Like Cannot Cut Like
Nothing can block a weapon made of a magical material — except for another weapon of the same material. Neither weapon will scratch or dent. Only a direct hit on the opponent will end the duel... which is also likely to kill them unless your Implausible Fencing Powers are good enough to disarm them without, y'know, "disarming" them. Needless to say, a character who thinks they have a Disc One Nuke in the form of their sword will be very unpleasantly surprised when facing an opponent who can even the odds. They may even get crushed if they've let their skills get rusty by relying on weapon's power. Such duels tend to prove suitably epic. A Sub-Trope of Mutual Disadvantage, of the "mutual invulnerability" variety. Contrast Takes One to Kill One, when only like can cut like.
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Anime and Manga
- Battle Angel Alita: Last Order: Monomolecular wire can be stopped by a stick made out of monomolecular material.
- In Black Butler, the Death Scythes can cut through anything. Naturally, a Death Scythe can't cut through a Death Scythe.
- Dansai Bunri no Crime Edge: No matter how powerful the Killing Good is, it cannot destroy another killing good.
- Beam sabers in Gundam are made up of a superheated Minovsky Particle plasma encased in I-fields. I-fields cannot block regular matter, so what allows beam sabers to cut through almost anything is the fact that hitting something with your saber is literally letting the plasma of the saber eat through whatever you're trying to cut. When two beam sabers make contact, the I-Fields from both swords are repelling the plasma of the other.
- Beam shields, introduced in the second Universal Century, rely on the same general principle, they're basically a beam saber elongated into a wide plane that can be used to block attacks. Incidentally, if enough force is applied to a beam shield (or enough concentrated pinpoint power in the case of the Variable Speed Beam Rifle), it's possible to collapse it and hit the target behind it.
- In Guyver, high frequency swords can't cut other high frequency swords, and when two high frequency weapons come into contact with each other, they make a deafening shriek.
- In Naruto, Sasuke can channel lightning through his sword to give it incredible cutting power as well as inflicting paralysis, but both effects are cancelled out when clashing with one of Killer Bee's sword also enhanced (albeit differently) by lightning.
- In Outlaw Star, Gene's caster shells could be blocked by firing another caster shell into them.
- Also subverted towards the finale, as there are some rare caster shells that will have abilities that only activate when blocked by another caster shell.
- In Samurai 7, the daito that the Samurai use can cut through anything from wood to Humongous Mecha...except other daito.
- Played straight in Shaman King, in that only Oversoul weapons can beat other Oversoul weapons (and Oversoul armor), but subverted when you get a character savvy enough to straight-up break their opponent's weapon, dispelling the Oversoul immediately.
- Gourry's energy blade sword in Slayers is eventually tested against another weapon that was created by the same people. When the two energy blades meet, they shriek. They then actually began to fuse together, and the energy released began to open a door to another dimension, wherein dwelt the god-killing Dark Star.
- In Kill Bill, the only thing that can withstand the cut of a sword forged by Hattori Hanzo... is another one.
- Lightsabers in Star Wars have this property, along with the ability to reflect blaster bolts. Only a small set of non-saber weapons can effectively parry them.
- According to ancillary materials, Sith lightsabers have properties that can occasionally cause the blade they are clashing with to deactivate. This explains why Obi-Wan's lightsaber flickered when he fought Vader in A New Hope.
- More ancillary materials give you the Barab ingot crystal. This crystal, when inserted into a saber, gives it the ability to occasionally flicker and pass through a blocking lightsaber without destroying it.
- The "reflect blaster bolts" property was touched upon in a novel, wherein an alien race was able to defeat Jedi because their blasters (permanently set to stun) had a different frequency. Lightsabers could be tuned to reflect these blaster bolts, but then they were rendered moot against regular blasters.
- Also done in Star Wars RPG games (starting with Knights of the Old Republic); when eventually acquiring a lightsaber, the player will soon find that, rather than the Infinity+1 Sword they may be expecting, it's essentially an upgraded, shiny sword that only chips away heath rather than a Game Breaker.
- The energy discs in TRON and TRON: Legacy can cut through programs like butter, but will rebound off each other harmlessly. Except at the end of Tron and Sark's last duel, where Sark tries a standard disc-block to deflect Tron's attack, but Tron is so much more skilled that his own disc flies right through Sark's (and Sark's head) as if the latter were wet tissue.
- In X2: X-Men United, Wolverine and Lady Deathstrike both have the adamantium claws/fingernails.
- X-Men Origins: Wolverine:
Wolverine faces Weapon XI, which has an adamantium katana.
William Stryker shoots Wolverine in the head with adamantium bullets (which is kinda justified, as a bullet, which is really fast, could have more impact than a blade). Doesn't kill him, but it does give him amnesia — presumably the shock of the impact gave him severe brain damage and his Healing Factor couldn't piece all his memories back together.
- In Harry Potter, Harry's and Voldemort's wands behave quite strangely when they try dueling each other, due to the wands being "twins", having feathers of the same phoenix (specifically, Dumbledore's pet phoenix Fawkes) as their respective cores.
- In the Incarnations of Immortality series, the highly magical Red Sword wielded by War cannot cut through Chrono's hourglass.
- Averted in Sergey Lukyanenko's A Lord from Planet Earth with the planar swords. While a planar (or monoatomic) sword cuts anything, it's similar to a lightsaber in this regard in that it passes through like knife through butter. When two planar swords meet, one will always cut through the other. Which one is determined by the angle of impact and the sharpness of each blade. This completely changes how people fights with these weapons. So no Blade Locks here. Unlike lightsabers, anyone can use a planar sword given sufficient training, and you can get them anywhere, although they're only produced on one planet.
- Played with in Michael Moorcock's Melnibone series; early on, Elric, armed with the runeblade Stormbringer, faces his cousin Yyrkoon, armed with Stormbringer's "sister" Mournblade. True to the trope, Stormbringer—which can cut through just about anything else it comes across—can't do that to its twin blade, and the cousins have an old-fashioned swordfight. As the battle progresses, Elric slowly masters Stormbringer, which has a will of its own; conversely, Yyrkoon allows himself to fall under Mournblade's control, with the result that Stormbringer and Elric neatly overpower Yyrkoon and Mournblade... casting Mournblade into hell, or at least another plane. That part's never really explained clearly.
- Shardblades in The Stormlight Archive can cut through any non-living substance except other Blades and Shardplate as easily as swinging them through the air (they pass straight through living flesh without leaving a mark, but kill the victim anyway). While Shardplate can only take a few hits from a Blade before shattering, the Blades can parry each other indefinitely without harm. This is probably because a Shardblade is the physical manifestation of a spren (it's complicated), while the origin of the armor is yet unrevealed but presumably different.
- The titular Sword of Truth has been known to cut through any weapon or armor - except for one case when a similarly enchanted blade is encountered.
- In the Wings of Fire series, the only antidote for a Rain Wing's corrosive venom is the venom of a sufficiently close blood relative.
- Shadowrun. To stop monofilament attacks (e.g. a monofilament sword), wear armor/clothing with monofilament threads woven into it.
- Warhammer 40,000: Played straight in the case of power weapons contacting other power weapons. The disruptive fields surrounding such weapons clash but typically prevent any damage to the physical component of the weapon itself.
- Singular Minecraft example: Golden Pickaxes cannot mine Gold Ore - all other non-wooden pickaxes can mine the ore they're made of, but gold, true to Real Life, is exceptionally weak.
- In Super Robot Wars Original Generation, one of the very few things that can stop Sanger Zonvolt's Colossal Blade is another Colossal Blade, wielded
- In Sword of the Stars, Mesonic Torpedoes will go through all Deflector Shields except for Meson Shields.
- Despite the utterly insane powers of the Red/Blue/Gold Keys of Umineko: When They Cry (including, but not limited to preemptively disallowing any sort of defensive concept) they can clash with each other. Note that in the Tips section it is mentioned that the Keys are limited in power for that Game Board, presumably to stop Dlanor swinging her sword twice and ending the story instantly.
- Blackstar has the Star Sword, and the Overlord has the Power Sword — two equally indestructible halves a superweapon called the Power Star (a few years before the Masters of the Universe toys would use the same gimmick).
- When Samurai Jack first meets the Scotsman, he's surprised to find his nigh-invulnerable sword cannot as much as scratch the Scott's similarly-enchanted weapon. The two spend the entire day fighting, and only succeed in exhausting themselves.
- When asked what made his sword special, the Scotsman replied "magic runes, laddie". This implies that his claymore, like Jack's katana, is endowed with magical properties, by gods or otherwise.
- Averted, strangely, in the case of diamonds. Bruting is a process wherein two diamonds are used to cut each other to desired shapes.
- Magnets: The sides with the same polarity will always repel each other.
- Unless brought together by a greater force. Then colliding into each other both chips chunks off (reducing the magnetic power), and reduces the magnetic power further through the interaction of their respective fields.
- This occurs in the sun. Stars basically start off as a collection of high energy protons, which collide with each other to create energy. The problem is, all protons are positive, and therefore, repel. The sun simply isn't hot enough to give the protons enough energy to overcome this propulsion, making it a seeming impossibility under classical physics. Quantum physics shows that protons could 'tunnel' through the repulsive effects of electromagnetism and collide with another proton often enough for fusion to work.
- Even more basic, the nucleus of an atom shouldn't be able to exist if the universe only had the Electromagnetic and Gravitational forces, considering it's basically a dense cluster of protons and neutrons (which add nothing). One of the four fundamental forces of nature, the strong force, keeps them that way.
- This is known as the Irresistable force paradox which dictates that if no one knows what will happen if an unstoppable force meets an immovable object, it's considered self-contradiction.