When magic spells call for blood, where do you cut? Right across your palm, of course. Because there certainly aren't any sensitive muscles or tendons there. Not an area packed full of pain receptors or anything. Plus it heals super fast, and you rarely use your hand anyway, right? Especially in the upcoming boss battle.
The main out-of-character reason for this trope is basically that it's the easiest kind of wound to fake.
- One can hide a packet of fake blood in one's hand to cut — and if one needs a lot of blood, one can hide the tubing up their sleeve.
- If one needs a close-up, a cut in the palm of a latex glove is easier to fake than a cut on the back or finger.
- If one needs to avoid close-ups, it's easier to keep the palm of one's hand out of view of the camera than the back or a finger.
Aside from all that, one has to consider the gravitas of the scene. Are there more practical body parts to nick for a bit of blood? Absolutely. Would it look nearly as dramatic or badass holding those body parts over some mystical chalice? Probably not.
- In Attack on Titan, Eren does this by biting into his palm, in order to transform into a titan. This is eventually further expounded later on in the story.
- In Naruto, people who need their own blood to cast jutsu will often cut/bite their fingertips instead.
- In Princess Tutu, Fakir revives Mytho's Cool Sword by cutting his hand, and pouring the blood from the wound into a fountain, and reciting a spell in Gratuitous German while dipping the blade into the bloody water.
- Subverted in Runaways. Nico's spell casting is done through The Staff Of One, which only emerges from her soul when blood is shed. The first time she actively attempts to summon it, she tries this. And she hates it. She would go on to come up with creative ways to shed blood that didn't involve cutting, from averting No Periods, Period to brushing her teeth until her gums bled.
- The Sandman: In Death: The Time Of Your Life, blood from a cut palm is used to power a spell to send the characters to the land of the dead and come back alive.
- No less than three times in Heart of Ashes, a palm is cut and blood dropped on someone else as part of a magic ritual. In the first chapter, Fankil does this to resurrect the recently perished Kathryn. In the same chapter, Andraya does this to awaken from deathlike coma the recently survived (and transformed into a man) Smaug at the cost of her fertility. The third time occurs during the climax when Fankil tries to fulfill the purpose he resurrected Kathryn for: locating the Door of Night through her influenced vision so that he can free his father Morgoth from the void.
- The Ultimate Evil: When Shendu is bound to his Other Valerie Payne in the rewritten reality, they slash each other's left palm and drink each other's blood as part of the ceremony. Their union leaves a permanent scar on each other's left palm that endures the restoration of reality.
- In the film The Brother From Another Planet, the protagonist uses a shard of glass to cut his palm in order to use his blood to write a message on a graffiti-covered wall. However, it's no big deal to him, as he is an alien who heals easily and quickly.
- Done in Deep Blue Sea when blood is needed to attract the sharks' attention.
- In The Hangover, Alan tries this to start a blood pact, freaking out everyone else.
- In Hellraiser, Larry cuts his hand on a nail and drips blood on the attic floor. Underneath lie the hidden remnants of Frank's body which the blood partially restores back to life and consciousness.
- The Client does this to both himself and Jay in Kill List while Jay is signing a contract.
- In the extended version of The Lord of the Rings: The Two Towers a human swears fealty to Saruman. When asked to prove it, he slices his palm open and says that they will die for Saruman.
- In National Treasure, Ben cuts his palm then presses it to the handle of the carved pipe they've found, so they can press the bloody pipe on a piece of paper and see what the words on it are. He couldn't find ink in the arctic.
- Done twice in Pirates of the Caribbean: The Curse of the Black Pearl, when blood is needed to break the curse. The first time, Barbossa does it to Elizabeth. Will does it to himself the second time, having to hurry things along before Barbossa killed either Elizabeth or Jack, who had just done it himself before throwing the medallion to Will.
- In Practical Magic, Sally and Gillian slice their palms and clasp hands to cure Gillian's Demonic Possession. It also conveniently works as a Curse Escape Clause.
- Robin Hood: Prince of Thieves has Robin slice open his palm during his oath to avenge his father, then he wraps it with a rag and continues battling for the rest of the film seemingly without pain or disability.
- At the beginning of Rob Roy Rob Roy is challenged to a friendly duel to the first cut but doesn't feel like participating, so he purposely slices his palm on the other guy's sword in order to lose. This becomes a Chekhov's Gun when in the climactic duel he grabs the other guy's sword with his bare hand in order to take it out of play.
- In Star Trek: Nemesis, Shinzon cuts open his palm to provide a blood sample for our heroes. It reveals that he's a clone of Captain Picard.
- In Aztec, Chimali's signature is the slap of his bloodied hand. This appears as a recurring theme throughout the novel. It's fitting, because Chimali becomes Mixtli's sworn enemy, and whenever he's done evil to Mixtli's loved ones, he leaves a handprint where Mixtli can find it.
- Mirai from Beyond the Boundary crosses this over with Wound That Will Not Heal: her main offensive power is manipulating her blood to form weapons and the like, and her source of blood is her palm. Her hand is almost always in a bandage to cover it up.
- In The Bishop's Heir, Kelson recalls the day he and Dhugal became foster brothers. Each of them used the other's dagger to slice open a palm, then they clasped their wounded hands together and Dhugal bound them with a handkerchief before they swore their oaths. Years later, they both retain faint scars across their palms.
- In Andre Norton's Dread Companion, Kilda has to bleed to get the grass that can act as a guide. She even has to pull it by the bleeding hand.
- Zig-zagged in Harry Potter and the Half-Blood Prince when Harry and Dumbledore encounter a door that requires a blood sacrifice. Upon entering, Dumbledore cuts his hand — but it happens to be the hand that is dead from a curse, so it's not as inconvenient as it normally would've been. When they leave again, Harry uses blood from the palm of his hand — which he had accidentally scraped on a rock earlier.
- In Mercedes Lackey's Heralds of Valdemar series, a mage who has become a reluctant Fisher King does this to mix his blood with the soil when he adds to his territory. Because of his connection to the earth, the cut heals very quickly.
- Casting spells with Wild Magic in The Obsidian Trilogy by Mercedes Lackey and James Mallory requires a blood sacrifice, usually done by a simple nick on the hand. However, in When Darkness Falls Idalia slashes her palm open with a dagger to summon the Starry Hunt to help battle the demons.
- Justified in On Stranger Tides, in which Shandy has to draw blood from his sword-hand's palm to maintain the mystical connection between his blade and his bloodstream.
- Princesses of the Pizza Parlor: From Princesses in the Darkest Depths: For some witchy, Blood Magic:
This was perhaps the witchiest thing she'd ever done [...] she took her personal knife and ran it over the palm of her hand.
- Averted in Sword of Truth. Richard goes for the inside of his elbow instead.
- This is the means Tenskwa-Tawa uses to produce blood — and pain — to work large magics in The Tales of Alvin Maker, and Alvin learns to do the same; for added messianic symbolism, when large quantities of blood are needed, like Tenskwa-Tawa walking onto Lake Michigan to raise a crystal tower inside a tornado, they pierce their feet too.
- Wulfrik regularly does this as his enchanted ship only responds to his blood. On occasion, he does it so hard blood bubbles from between his clenched fingers, but that's entirely normal behavior for Norscans.
- Angel: Darla cuts her palm when a shaman needs a blood sample for a spell after he is reluctant to do it to her.
- Done at least twice in Babylon 5, by Narn. Once by G'Kar to make a point to Vir, once by Ta'Lon to satisfy a tradition that his sword must draw blood before being sheathed again. Seems to be a Narn thing.
- Boardwalk Empire. Season 5 has Charlie "Lucky" Luciano's Mafia initiation.
- Game of Thrones.
- Beric Dondarrion does this before challenging Sandor Clegane to Trial by Combat, which seems a foolish thing to do until it causes his sword to burst into flame due to implied Blood Magic. Sandor as it happens is terrified of fire.
- Xaro Xhoan Daxos cuts his hand and shows the blood as part of his oath to stand for Daenerys and her people. He's lying.
- Averted when a teenaged Cersei Lannister consults a witch — she cuts her thumb for the required Blood Magic.
- Krypton has Superman's grandfather, Seg-El, opening his grandfather's Fortress of Solitude on Krypton using the blood of an El. It forms the familiar sigil of the House of El.
- In an episode of Law & Order: Criminal Intent, Goren cuts his hand in order to show a suspect is too squeamish about blood to have committed a very bloody crime.
- In one episode of Lie to Me, Cal slices open his hand to show that the guy they are interviewing has a phobia to blood.
- The Magicians episode "Divine Elimination": Margo slices open Elliot's palm to summon Ember using a blood-stone.
- Star Trek: The Original Series episode "A Private Little War". When Nona uses the Mahko root to heal Kirk, her husband Tyree cuts her hand first so that her blood joins with the root before it's applied to Kirk's wound.
- Supernatural uses this frequently, often to prove that someone is human by showing that they don't have Alien Blood or react to various Kryptonite Factors. Season 3 Episode 9 Malleus Maleficarum has a witch slicing her hand open in the first few minutes.
- Star Trek: The Next Generation and Star Trek: Deep Space Nine establish this as a Klingon habit. If they're putting on any kind of ceremony, it's a safe bet that shedding some blood this way is going to be part of it.
Admiral Ross: It still stings.
- In the Season 4 premiere The Way Of The Warrior, when General Martok arrives on Deep Space Nine, he demands a meeting with Captain Sisko and Major Kira. He then demands that they all prove that they're not Changeling infiltrators, and demonstrates by cutting his own palm and letting a few drops of blood fall onto the tablenote before handing his knife to Sisko. Sisko and Kira follow suit, which satisfies Martok, and they proceed with the meeting. However, it turns out later that "General Martok" WAS a Changeling infiltrator. They'd already figured out a way to fool the test (likely by hiding blood packs inside their bodies), and it was only for show.
- In the Season 7 episode When It Rains, Captain Sisko and Admiral Ross have been invited to a ceremony where Chancellor Gowron presents a commendation to General Martoknote . After watching both Martok and Gowron cut their palms, Sisko glumly says "We're next." Ross chuckles until he looks over at Sisko and sees he's serious. Cut to the party after the ceremony, and they both have drinks in one hand, and bandages around the other.
Captain Sisko: That's what the bloodwine's for.
- V (1983) had a scene where someone approaching La Résistance for help did this to prove they were not a Visitor.
- The Blue Öyster Cult song "Unknown Tongues" is about a Catholic schoolgirl called Margaret, who lies in bed at night contemplating the mysteries of Glossolalia and the Holy Stigmata (i.e.: the voices in her head tell her to self-harm with a razor blade):
And then she took/Her father's razor;
And watched it cut into her palm;
She put her hand up to her mouth
To taste the blood, so holy and warm....
- S. J. Tucker's song "Valkyrie Daughter" has this when the soldier makes his deal with the goddess Hel to forfeit his place in Valhalla so that his daughter can go from Niflheim to Valhalla and become a Valkyrie.
The soldier's eyes clouded... But he cut his own palm,
And signed all his honor away.
- Call of Cthulhu campaign Spawn of Azathoth. During a Cthulhu Mythos ritual each participant must cut the palm of his hand and smear the blood over a severed hand, which animates and acts like the planchette of an Ouija board to point out symbols and provide a message.
- Dungeons & Dragons. In module CM1 "Test of the Warlords", the PC's can participate in the ritual of blood sharing with a barbarian chief. They do so by cutting their palms and clasping hands, thus mixing their blood.
- GURPS Illuminati. The sidebar titled "Blood Samples" describes Illuminati initiation procedures that require using the applicant's blood, such as signing a contract, filling a chalice, etc. The accompanying illustration shows a knife cutting the palm of a person's hand.
- Averted in Dawn of War: Soulstorm. Where most units capture points by standing at attention/prayer, the Dark Eldar slash their wrists and hold the bleeding hand up in salute.
- Vergil in Devil May Cry 3 does this as a part of the ceremony to break the spell on the Temen-ni-gru, using his katana Yamato. In his case, it's doubtful this would slow him down.
- In Diablo II, The Necromancer has a Curse spell called "Life Tap." According to Flavour Text in the guidebook, he slits his palms to allow Life Energy to flow through them.
- In Dragon Age: Origins, Blood Magic always has the somatic component of cutting the palm. By stabbing it with a dagger.
- In The Elder Scrolls V: Skyrim, at one point a door needs to be opened with the blood of the Dragonborn. They cut the palm of their hand to do so.
- An unintentional version happens in the Dawnguard DLC: The Dragonborn investigates an ancient tomb and, when s/he presses what looks like a button, a spike pops up and pierces their hand, allowing them to open the blood seal keeping Serana entombed.
- In Destiny 2: Season of the Chosen, this is used as part of a ritual, but not a magical one. As part of an armistice between Empress Caiatl's Cabal and the Guardians, there is meant to be this between Caiatl and Commander Zavala (on behalf of the Guardians), a symbolic gesture, "Let this be the last blood spilt between our people." It only gets as far as Caiatl doing her half before conspirators attempt to assassinate Zavala.
- The Dragon Prince: The mage Viren is highly implied to have done this as part of a spell, as instructed by the mysterious elf Aaravos.
- The Road to El Dorado: The wicked high priest Tzekel-Kan slices his palm to smear blood upon a stone relief of one of the two founding gods of El Dorado. He does this to emphasize that he has seen through Tulio and Miguel's masquerade. "Gods. Don't. Bleed."