[[quoteright:326:[[VideoGame/TheLegendOfZeldaOracleGames http://static.tvtropes.org/pmwiki/pub/images/dualworld_2458.png]]]]
[[caption-width-right:326:Left = Past era. Right = Present era.]]

A video game that features the concept of the player travelling between two (or more) different versions of the same "world" -- often in the form of [[TimeTravel the past or future]], a DarkWorld, or a [[AnotherDimension parallel universe]].

The game will present puzzles and obstacles to the player that can ''only'' be solved in one version of the world or another, or which otherwise require travel between the two worlds to overcome. Specific gameplay features will vary depending on the game's design and other thematic choices, but there are several common techniques:
* Having a particular transition point (or points) to prevent the player from changing too freely (a portal between the worlds, a time machine)
* Closed off paths in one world are unobstructed in the other, and vice versa (a wall that's crumbled away in the future, walls are immaterial in the shadow world)
* Character's actions in one world directly effect the other (a seed planted in the present grows into a climbable tree in the future)
* Having directions or objective indicators existing in only one world (demonic markers in the hell world, signs that are still fresh in the past).
* Certain enemies may only appear in one of the worlds (demons only occur in the demon world, roaming zombies only appear in the future after the ZombieApocalypse)
* One world may inflict damage on the player limiting how long they can stay there (acidic atmosphere erodes your armour, the bright sunlight of higher realms damages the undead hero)

See LayeredWorld for the more general concept. Likely to feature an AlternateWorldMap.

!! Examples:



[[folder: Video Games ]]

[[folder:Action Adventure]]
* ''VideoGame/CastlevaniaHarmonyOfDissonance'' does this. It's a bit of a reveal that the second castle exists though - then you warp back and forth between the two castles. This makes backtracking very annoying, especially before TheReveal of the two castles' natures. Near the end, you have to locate all six of Dracula's relics, hidden away in both castles. You'll have likely found two or three initially though. Where you fight at the end also determines the ending.
* ''VideoGame/{{Guacamelee}}'' shifts between the worlds of the living and the dead, and several platforms and enemies only exist in one or the other.
* ''VideoGame/LegacyOfKain: Soul Reaver'' featured an original take on this with the material realm/spectral plane mechanic: Since the player character is an undead wraith, he is a native of the spectral plane, his energy drains constantly in the material realm (unless he has maximum health, and the titular Soul Reaver on-hand), and losing all of his health kicks him back into the spectral plane, although he can shed his material form at any time. However, travelling back to the material realm requires the player to find a conduit and be at full health. In the spectral plane, time stands still, so the player can't open doors or push or pull items, but with a certain skill they can at least transition through gratings. In addition, the scenery in the spectral plane is twisted and contorted, so what was once an impassable gap in the material realm can turn into a doable jumping puzzle in the spectral plane. The duality factored into the combat as well: The player could find dangerous wraiths in the spectral plane, and revive their bodies in the material realm, where they were even more difficult to fight.
* ''Franchise/TheLegendOfZelda'':
** ''VideoGame/TheLegendOfZeldaALinkToThePast'' started this trend in the series. Link stumbles across a portal to the DarkWorld just before [[DiscOneFinalDungeon what seems to be the final dungeon]]. This is one of many portals in Hyrule which trap people in the Dark World and [[BalefulPolymorph warp their bodies]]. Luckily, Link gets the Magic Mirror right before he enters, and uses it to return to Hyrule. The mirror is used extensively in the second half of the game as Link finds more portals and collects crystals from the Dark World dungeons.
** In ''VideoGame/TheLegendOfZeldaOcarinaOfTime'', Link can travel [[TimeSkip seven years]] forwards and backwards in time by using the Temple of Time. Once you learn some [[MagicMusic songs]], you'll need to return to the past to get some items, and then go back to the future to use them as an adult. Link can also plant seeds in the past to gain access to items in the future, and [[GuideDangIt finish a few other fun sidequests]].
** In ''VideoGame/TheLegendOfZeldaOracleGames'':
*** In ''Ages'', pictured above, Link learns [[MagicMusic songs that let him use time portals]] scattered across Labrynna to travel centuries into the past. It's very integral to the plot, because a sorceress has travelled to the past and is mucking up the timeline. Link has to [[PlotCoupon collect items from dungeons]] in the past and present in order to defeat her. Later he gains the ability to travel back and forth more freely.
*** The companion game, ''Seasons'', features season-changing gameplay. Every part of the world map is mapped differently for each of the four seasons. Spring makes big flowers grow that shoot you up cliffs, water dries up and vines grow up walls in the summer, leaves cover pits and mushrooms soften in the fall, and water freezes and snow piles up in the winter.
** ''VideoGame/TheLegendOfZeldaFourSwordsAdventures'' also features the Dark World, but it doesn't cover the entire world. Link can collect Moon Pearls in some levels to open portals, which are used to bypass obstacles and solve puzzles. It comes up fairly often, but it isn't central to the game.
** The concept in ''VideoGame/TheLegendOfZeldaTheMinishCap'' is different in theory, but the same in execution. The two worlds in this game are the Human World and the Minish World, which aren't different worlds at all. The Minish are [[{{Lilliputians}} about an inch tall]] and live alongside the humans, unnoticed. It's really the same thing gameplay-wise: Link can only shrink at certain places, changing things at normal size open up paths when shrunk and vice versa, and certain enemies are only encountered while shrunk. The game takes that last part a step further: you can encounter normal Zelda enemies in the Minish realm, but while they die in one or two hits at normal size, they become massive bosses once you're an inch tall.
** Oddly, ''VideoGame/TheLegendOfZeldaTwilightPrincess'' averts this trope. The world doesn't change much when it's under the Twilight Realm's influence, but all the [=NPCs=] and monsters do. The Twilight Realm itself does not mirror Hyrule at all, and acts instead as the penultimate dungeon.
** ''VideoGame/TheLegendOfZeldaSkywardSword'' includes the Silent Realms, areas based on parts of the game world that make you go through a StealthBasedMission to get needed items. There's also the entire region of Lanayru, the location of the Temple of Time. There are minerals here that can create time portals to the past when Link strikes them. They usually only affect the immediate area, but Skipper's Sandship carries the most powerful one, able to stretch its effects at least as far as the horizon, effectively giving the place a true present-mode and a past-mode.
** Lorule is the new DarkWorld of ''VideoGame/TheLegendOfZeldaALinkBetweenWorlds'', as its title might hint at. You travel between Hyrule and Lorule through special cracks, which require you to turn into a painting to pass through.
* ''VideoGame/PrinceOfPersiaWarriorWithin'' had the Prince moving between the Island of Time's past and present. In the past, he could access rooms that were destroyed in the present and meet characters that were long dead by that time. In the present, he could use the damage to the island to access areas that are normally inaccessible in the past.

[[folder:Adventure Game]]
* ''VideoGame/DayOfTheTentacle'' features a novel variation. The three characters are trapped in three different time periods and have to work together, [[BagOfSharing using items from different time periods]] [[JustifiedTrope by passing them through]] a time-traveling toilet.
* ''VideoGame/TheLongestJourney'' was originally pitched as a PlatformGame with Dual World Gameplay, whose protagonist would have to jump between fantasy and cyberpunk versions of the same levels to get through them. However, under the influence of the creative director, it has, over time, evolved into a sprawling AdventureGame epic with complex cosmology based on the contrast between the scientific world we know and its magical alternate reality. In other words, the dual world idea moved from the realm of gameplay to the realm of story and never returned.
* ''VideoGame/MysteryOfMortlakeMansion'' features a real-world mansion and its "shadowy" counterpart, between which the protagonist must frequently travel.
* ''VideoGame/TheSplitting'' has two worlds, the normal world and the mirror world. The player character has to figure out how to progress by using objects from world in the other or modifying something in the normal world so the mirror world changes to match.

[[folder:Fighting Game]]
* Happens in ''The Lake'' stage in [[VideoGame/SuperSmashBros The Subspace Emissary]]. Reused in [[TheVeryDefinitelyFinalDungeon The Great Maze]] as well.

[[folder:First Person Shooter]]
* The Vault of Glass in ''VideoGame/{{Destiny}}'' has this when you're facing Atheon, the portals are the same place but in the past and future.
* In the ''Opposing Force'' expansion of ''VideoGame/HalfLife1'', Adrian Shepherd gets a portal gun that lets him jump between Xen and Earth at any time. This is usually a good idea if you're in a tight spot, because Xen tends to be less hostile than Black Mesa. You can access Xen from any point by just using the gun's alt fire, but you always return to a fixed point in Black Mesa (varies with the map you're on). What awaits you in Xen varies, from some supplies, a healing pool, to a very, very long drop.
* ''VideoGame/MetroidPrime2Echoes'' has Samus travel between Aether and Dark Aether. The planet was split by the impact of a [[spoiler:Phazon]] meteor, and the inhabitants of Dark Aether started an invasion. Even worse, [[ApocalypseHow Aether is in danger of being destroyed]] because the Ing stole critical technology from the native Luminoth. [[LateToTheTragedy Samus finds portals made by the Luminoth]] that let her travel there to set things right. Because Dark Aether isn't as interconnected, each level's DarkWorld feels like a claustrophobic, twisted splinter of reality. And the atmosphere of Dark Aether [[EverythingIsTryingToKillYou will eat your suit, just like everything else there]]. Fortunately your suit gets an upgrade somewhat early in the game that makes it more resistant to Dark Aether's atmosphere, and another upgrade near the end of the game that makes it completely immune.
* ''VideoGame/Prey2006'' lets you leave your body temporarily to walk around in the spirit realm. This lets you walk through force fields, kill enemies with arrows, and do some other nifty stuff.
* ''TNT: Evilution'' does this in Map 4, which has a regular world and a [[DarkWorld Dark]] MirrorWorld. Subverted to an extent, as the latter ends up being superfluous to completing it.
* ''VideoGame/{{Wolfenstein|2009}}'' (2009) has the Veil, a strange area between dimensions that can alter the appearance of the environment (i.e. brand new airplanes appear derelict) and is what powers the Amulet's abilities.
* ''VideoGame/Titanfall2'' features a stage where Jack and BT investigate a ruined IMC facility to determine it's cause. As Jack explores the base, he finds time becoming unstable. Eventually, he gains a device that allows him to shift back and forth to the past, where it eventually becomes clear that [[PredestinationParadox he was at least partially responsible]]. The stage itself features several moments where Jack often has to time-shift between {{Parkour}} jumps and in and out of battles, culminating in a protracted firefight against a squad of soldiers in the past ''and'' a pack of hungry alien wolves in the present.

* ''VideoGame/MasterOfMagic'' has Arcanus and Myrror. The latter is a MirrorUniverse with other races and more magic. Units can go back and forth via towers, "plane walk" spell and some on their own ability. The goal is to conquer both, anyway.
* ''VideoGame/AgeOfWonders: Shadow Magic'' one-ups this by having battles take place in up to ''three'' parallel worlds: the Surface world has varied terrain and mimics the real world (albeit with PatchworkMap tendancies), the Underground is a vast warren of caves that restricts movement and visibility for most races (giving an edge to natural burrower factions), and the Shadow world is a bizarre astral plane that allows rapid movement, but inflicts a debilitating shadow sickness on most races (giving a huge combat advantage to units immune to this effect).
* ''VideoGame/{{Civilization}} II: Test of Time'' has two campaigns that each include multiple parallel "world" that can be played simultaneously. The science fiction campaign has Funestis (Earth-like), orbital platforms (SpaceZone), Naumachia (rocky planet with no water), and Nova ([[FloatingContinent floating platforms in a gas giant]]). The fantasy campaign and the Midgard scenario have the Surface World, the Underground World, the Cloud World, and the Undersea World.

[[folder:Party Games]]
* ''VideoGame/MarioParty'' has a few boards that change state. All of the boards in ''VideoGame/MarioParty 6'' undergo a day/night cycle.

* Click Clock Wood in ''VideoGame/BanjoKazooie'' is a vertically sprawling forest which you must enter in four different seasons of weather, via four separate doors. They differ in terms of aesthetic, enemies, and accessible areas, and some puzzles must be solved by hopping among the seasons, affecting future seasons. Such a big challenge is fitting for the last main level.
* The first stage in the "Milky Way Wishes" part of ''VideoGame/KirbySuperStar ([[UpdatedRerelease Ultra]])'' Is a stage with doors leading to the same area, only in a different season. The object was to traverse the stage in the right seasons to bypass obstacles.
* ''Franchise/SuperMarioBros'':
** ''VideoGame/SuperMario64'' features Tiny-Huge Island, where Mario can go through pipes to change size. As a giant, Mario can easily jump up cliffs and doesn't need to worry much about enemies. As a midget, Mario can ride on breezes and enter narrow passages. This is implemented by two different maps of the same island. ''VideoGame/SuperMarioBros3'' has a similar level, where going through doors shrinks the normally enormous enemies to normal size. The environment remains big, however.
** Happens strangely in ''[[VideoGame/MarioAndLuigiBowsersInsideStory Bowser's Inside Story]]''. In some places, Bowser can do things that change some aspects of his internal workings. If he drinks water, some areas become covered in such, allowing the Bros to swim. Standing in front of an X-ray reveals some passages. And so on.
** A variation is used in ''VideoGame/MarioAndLuigiDreamTeam''. While Pi'illo Island and the Dream World don't have nearly identical maps like most examples of this trope do (largely because the former having ThreeQuartersView and latter having SideView make this impossible), the part of Pi'illo Island where the Dream World is accessed affects the latter's layout. For instance, if Luigi falls asleep on a pillow where a windmill is in the vicinity, that windmill will appear in the dream (though with a much different positioning and use).
* Every stage in ''VideoGame/SonicTheHedgehogCD'' has four variants: Past, Present, Good Future, and BadFuture. Each of these variants has slightly different stage design, and some stage gimmicks work differently between the four of them. While in the past, Sonic can destroy a Metal Sonic projection to free the animals, upon which they'll leap around in the present and future; and destroy a robot generator, which renders the stage free of enemies in other time periods and changes the future from bad to good.
* A variation of the trope is present throughout ''VideoGame/SonicUnleashed''. Not only do the "dual-worlds" in question (daytime/nighttime) have completely different level designs, but they also have [[PlatformGame different]] [[BeatEmUp gameplay]] styles between them.
* ''VideoGame/SpyroShadowLegacy'' pits the eponymous hero against enemies from the Shadow Realm. These creatures are impervious to standard attacks, which makes it easy for them to capture just about everyone; even [[TheChosenOne Spyro himself]] is initially powerless against them. After undergoing a crash course in new combat techniques, Spyro hops back and forth constantly between the material and shadow planes; some bosses require you to switch worlds multiple times to bring them down.
* ''VideoGame/WarioLand 3'' has a day/night cycle integrated into the entire game, with each level mapped twice for day and night. [[GuideDangIt Many treasures cannot be accessed at the wrong time]]. During the day, [[{{Mooks}} monsters patrol everywhere]] and doors in cities are open. At night, [[MechaMooks robots prowl where enemies were]], and the hungry dead stalk the dark places. Near the end of the game, you get an item that lets you change the time of day on the world map without having to enter a level.

* ''SugarWiki/{{Eversion}}'' actually has more than one layer: there are 8 in total, but per level, you'll only be using 2 or 3 of them. At different layers, different objects may be obstacles or even enemies. In the lower levels, clouds can be platforms. In the highest levels bushes are obstacles, go lower and they are death traps, beyond that they are just dead and background art. Enemies are deadly to different degrees with different one optimised for different levels.
* ''VideoGame/HeartStar'' has a gimmick based around this, where pressing space cycles between the two worlds. The player's job is to get the two characters to the goal by cycling through the worlds.

[[folder:Racing Games]]
* ''VideoGame/MarioKart8'': The Animal Crossing DLC stage has four different variants that move between seasons, but also between the time of day. The spring track is in the morning, summer occurs during the day, autumn is in the evening, and winter takes place at night.

* The second ''VideoGame/{{Boktai}}'' game features the BonusDungeon House of Time, where you travel between past and present to advance.
* ''VideoGame/ChronoTrigger'' does this with seven different time periods, accessible via fixed [[PortalToThePast time gates]]. After you find [[CoolAirship The Epoch]], you can time travel anywhere on the world map. The two most closely linked time periods (and thus most true to the spirit of this trope) are the Modern (1000 AD) and Medieval (600 AD) eras.
* ''VideoGame/ChronoCross'' has parallel worlds from a different timeline each. There is only one portal to travel between them, and you have to figure out your own way to navigate the world map in each. Despite being parallel worlds, one side can affect the other (e.g. cooling scorched ground on an island in one world allows plant life to grow in the other world).
* Technically turns up in ''VideoGame/DragonAgeII'', where the "other world" is simply Kirkwall at night.
* ''VideoGame/DragonQuestVI'' takes place [[spoiler: mostly]] in the Dream World and the Real World, which look very much alike, (though they have different layouts, some towns exist in both worlds) and you'll be traveling between them often.
* ''VideoGame/DragonQuestVII'' has the similar-looking present and past worlds.
* Final chapter of ''VideoGame/NeverwinterNights: Hordes of the Underdark'' features a ring that shifts you into a parallel world when equipped, which is required to pass some obstacles. ''Shadows of Undrentide'' also has a short plane-shifting sequence.
* ''VideoGame/NeverwinterNights2: Mask of the Betrayer'' does this. Things in the plane of shadows are shaped differently than in the material plane. Most notably the small temple of Kelemvor becomes the massive Death God's Vault. You can use portals to the plane of shadow to pass obstacles or find things that don't exist in the material plane, or find people hiding out in the plane of shadows.
* ''VideoGame/RadiantHistoria'' evokes a variation of this trope - instead of separate worlds, Stocke has to traverse two separate timelines, or "Histories". When you meet a road block in the "Standard History", you'll have to seek its solution in the "Alternate History", and vice versa.
* The ''[[RaidouKuzunohaVsTheSoullessArmy Raidou]] [[RaidouKuzunohaVsKingAbaddon Kuzunoha]]'' action RPG games have a normal world and a DarkWorld.
** Switching is entirely under the player's control but can be done only from one place. The Dark Capital is essentially a gloomy and overcast version of the regular capital. There are no people on the streets (normally); it's inhabited entirely by demons. Energy barriers called Dragon Gates block paths that are accessible in the normal version of the Capital, and you can only go between the regular Capital and the Dark Capital at certain shrines.
** The Dark Capital is useful to you in that it is your main source of demons to negotiate with, which gets you allies in battle. Refusing to visit it except strictly as necessary will leave you underpowered and ill-prepared for the challenges you'll face in the game.
** It's also necessary to the plot; oftentimes those kidnapped by demons wind up in the Dark Capital, and people harassed by demons usually require the problem to be traced back to the location's Dark Capital counterpart. As a Devil Summoner and a private investigator, that usually means it's Raidou's job (and thus, the player's) to do it.
* The Kabuki Cho prison in ''VideoGame/ShinMegamiTenseiIIINocturne'', where the other world is upside down, making it possible to get past [[InsurmountableWaistHeightFence the waist-height fences]].

* ''VideoGame/{{Minecraft}}'' has the normal world and [[FireAndBrimstoneHell the Nether]], accessible through obsidian portals when lit on fire. (This creates an inconvenience in SMP - survival mode multi-player - where having two maps hosted on the same server can be a major memory strain.) The Nether is smaller than the normal world, as ten meters in the Nether is eighty meters outside. This makes it convenient for fast travel, [[EverythingTryingToKillYou assuming you can travel safely]]. Later on, at full release, ''Minecraft'' introduced a third world, called "The End". It's a floating landmass, full of Endermen and one of the game's bosses. To get there, you need to fix an enderportal, found in a Stronghold (found in the main world), with items obtained in the Nether. There's also a GameMod that introduces ''VideoGame/TheAether''. It's a world made of floating islands and clouds. If you fall, it brings you back to the normal world. If you don't have a parachute, the fall will most likely kill you.

[[folder:Shoot 'em Up]]
* ''Chaos Field'' has the Order Field and Chaos Field, hence the name. In the latter, the player's weapons are more powerful, but the boss's attacks [[BulletHell increase in intensity]].

[[folder:Stealth Games]]
* In ''VideoGame/SheepDogNWolf'', Level 8 and the second secret level have a time-travel gimmick. Using a special watch item will teleport you back to an ancient version of the mountain forest, where you can plant seeds that grow into climbable trees or move boulders that become rocks you can hide behind once you return to the present. Just look out for the lava and dragons.

[[folder:Survival Horror]]
* The ''Franchise/SilentHill'' series is probably one of the better-known examples of gameplay involving switching back and forth between a {{Dark World}} and a... comparatively normal world, although it generally isn't really under the player's control. [[VideoGame/SilentHillOrigins One of the newer, not as well received games]], gave the player complete control.
* ''[[VideoGame/ChzoMythos Trilby's Notes]]'' has a normal world/dark world mechanic heavily inspired by ''Franchise/SilentHill'' -- the player can switch at will to a certain extent and often encountered the "path open in another world" technique, although the DarkWorld does pop up unexpectedly sometimes.

[[folder:Turn Based Strategy]]
* The X-Dimension stages in ''Franchise/{{Disgaea}}'' are... sort of like this. Basically they're modified versions of the same stages from the story, but with higher level enemies, geo-block puzzles and occasionally other special conditions.


[[folder: Other Games ]]

[[folder:Tabletop Games]]
* The ''TabletopGame/DungeonsAndDragons'' cosmology.
** Classical version expanded and expounded in ''{{Planescape}}'' has Ethereal and Astral planes to connect everything else to the Prime (worlds of mortals) and consequently have corresponding areas. Ethereal has Borders, contacting Prime and Inner planes and separated from Deep Ethereal by curtains. Also, shadow mages discern Shadow Fringe -- less dark and dangerous part of Demiplane of Shadow connecting to the shadows in Prime. So, ghosts dwell in the Border Ethereal, invisible but able to see everything in reality within a few feet. When they want to haunt someone or something, they can "manifest" by moving halfway between the planes. Magic users can become ethereal to float through walls, "make a short cut" via Shadow Fringe or move to another plane entirely.
** 3.5 edition has coterminous planes, which connect at a few points, and coexistent planes, which overlap.
** Adventure I10 ''Ravenloft II: The House on Gryphon Hill''. If the [=DM=] had module I6 ''Ravenloft'', the {{PC}}s could switch back and forth between modules, fighting Strahd in both. It's possible for the {{PC}}s to find items or information in one world that could help them in the other.
** ''Birthright'' campaign setting. Halflings originally came from the Shadow World but fled to Cerilia when a force of evil corrupted their homeland. They and other Cerilian races can travel there if necessary.
** Streamlined in 4e, with the Feywild and Shadowfell being the brighter and darker version of the world, respectively.
* ''TabletopGame/MagicTheGathering'' has done this a few times, with suspend (exile with counters, removing one during each upkeep, and playing it when they're all removed), phasing (put the card and everything attached to it in a special zone at the end of turn, or whenever it "phases out", only to put it back in play at the beginning), shadow (creatures with shadow cannot block or be blocked by creatures without shadow, seen as another world), and lots of things like Parallax Wave (exile cards temporarily).
* ''TabletopGame/{{Exalted}}'' has the Underworld, which is a dark reflection of Creation; some necromantic spells allow you to instantly step between corresponding points on the two, and the dark Labyrinth beneath the Underworld also allows it to be used for fast travel. There's also Yu-Shan, which is a celestial copy of the Blessed Isle that links to Creation at several points, and is covered with canals for high-speed travel. There's many other worlds, like Autochthonia and Malfeas, but the Underworld and Yu-Shan are the only ones that explicitly map to Creation.
* Several of the monster types in White Wolf's ''TabletopGame/OldWorldOfDarkness'' have access to the Umbra, a spirit world containing a duplicate of the entire world. The Garou in particular are known for jumping into the Umbra to study or fix problems popping up in the physical world. From their enemies' perspectives, they are also known for jumping ''out'' of the Umbra, resulting in a [[LightningBruiser ten-foot-wolfman suddenly appearing from thin air to tear your head off with little to no warning sign at all.]]
* WhiteWolf loves this trope as an explanation for the creepy crawlies of the gamelines in ''TabletopGame/NewWorldOfDarkness'':
** ''TabletopGame/WerewolfTheForsaken'': The player characters are part-human, part-wolf-spirit that can easily sidestep between this world and the Spirit World (called 'Shadow'). Any permanent-ish solution to the world's [[CrapsackWorld many]] woes usually need things to be done on both side of the Gauntlet.
** ''TabletopGame/GeistTheSinEaters'': The same, but replace Werewolf with Sin-Eaters and Spirit World with The Underworld.
** ''TabletopGame/ChangelingTheLost'': Changelings have Arcadia and the Hedge, but they do not map onto the real world as much. And Arcadia is the realm of the fae which Changelings are not too keen on getting to.

[[folder:Board Games]]
* This is the entire concept of a [[HouseRules variant]] of chess called Alice chess. A piece on Board A moves to a space on Board B.