The double-headed coin is Two-Face's favored possession and the tool that aids his weapon of choice, making decisions.
After Harvey Dent, the lawyer who prosecuted mobster, Sal Maroni, was splashed in the face with acid, he became Two-Face. He then took Maroni's lucky double-headed coin. Most famously, Two-Face uses the two-headed silver dollar, one side of which is scarred, the other side clean. Flipping the coin dictates Dent when making any significant life decisions. If the coin lands on the scarred side, Two-Face will pursue acts of evil. If the coin lands on the unmarked side, he is compelled to commit acts of good.
Two-Face often defers to his coin in choices of life and death, to the point that he will rely on the coin even if the subsequent flip jeopardizes his own plans. He once collaborated with the Joker and the Penguin to poison Batman, but when Robin suggested a coin-toss to decide the final outcome of the plan, Two-Face attacked the Penguin to get the antidote. During No Man's Land, Two-Face put James Gordon on trial for his 'illegal' alliance with Two-Face during an earlier plan to protect Gordon's territory, but when Renee Montoya noted that Gordon needed a defense, Two-Face tossed a coin to confirm that Harvey Dent would act as Gordon's defence lawyer, Dent's cross-examination of himself leading to Gordon's acquittal.
At one point, Two-Face was so dependent on the coin for any of his criminal acts that he could be easily defeated by taking the coin away from him or preventing him from seeing the result of the flip. Harvey Kent (Earth-Two) was usually shown at this level of dependency. Dent has since lessened his dependency on the coin flip to whether a situation should be implemented, rather than each individual act; for example, during Prodigal, he set up a plan where he would disrupt all criminal records of those currently awaiting full sentencing, but still relied on a coin toss to decide whether he put the plan into action by flicking the final switch or not even after setting everything up for the plan to go into action.
Two-Face's signature coin appeared in Batman Forever. Batman easily defeated Two-Face by throwing up multiple coins and silver dollars while Two-Face was in the middle of flipping his coin. Instead of being totally dependent on the coin for what he should do next, on at least one occasion, when he was faced with an opportunity to shoot Bruce Wayne after he learned that Bruce was Batman and had rendered him unconscious in an attack on Wayne Manor, Dent decided what he wanted to do himself and just kept flipping the coin until he achieved his desired result (although the Riddler stopped him from actually shooting Wayne, as he preferred to defeat their foe more directly rather than just catching him by surprise).
Later on when the Riddler was defeated, Dent seized the opportunity to kill Batman once and for all, but the Dark Knight made him decide with his coin once again. When he tossed it, Batman threw a handful of identical decoys at him, confusing Two-Face, making him lose his balance and fall to his death. However, as the criminal sunk to his watery grave, the coin landed one last time in his open hand, with the unscarred side up.
Two-Face flipped his own two-headed coin whenever he had to make a decision. Before his accident, Harvey Dent used the coin to trick criminals into confessing, such as when he kidnapped Thomas Schiff and informed him, "Heads, you get to keep your head. Tails, not so lucky". But because both sides were heads, Dent never killed. After Dent's accident that turned him into Two-Face, one side of the coin was badly burnt while the other side was fine. Two-Face then used the coin to actually decide whether his target would live or die. Dent stated that the coin was a keepsake from his deceased father, which he implied was also the reason that he managed to ask Rachel Dawes out.
The Dark Knight novelization expanded a bit on the coin's backstory, where it was revealed to have been used in a manner similar to the comics' rendition of Dent's childhood as a means for his father to give his son a beating (with the coin always choosing in favor of having him beaten due to its double-headed nature). Dent's father's status as a former cop also prevented anyone from reporting to the authorities of his evident child abuse, which consequentially led to Dent's severe hatred for corrupt cops, especially the ones in Commissioner Gordon's employment.
In Gotham, the pre-disfigured Harvey Dent uses the double-headed coin toss in talks with young offenders and other minor criminals, claiming that he will let them pick a side and then leave it up to fate to determine if he sends them to jail or lets them off with promises that they won't repeat their actions, musing that the strategy works as most teenagers pick 'Heads'.
The Dark Knight Returns
A decade after Batman's retirement, presumed 'cured' after plastic surgery repairs his face, Harvey Dent instead suffers a final psychotic break, scarring both sides of his coin to reflect his new darker approach to crime.
Batman: Crimson Mist
In Earth-43, Two-Face relied on a coin-toss to determine how he, Killer Croc and their gang would respond to the threat of the vampire Batman, stating when the coin landed scarred side up that they would 'extend this alliance to [their] opposites'. Croc noted that he felt the coin should be good side up to make that decision, but Two-Face said that he would see what that meant later. Working with James Gordon and Alfred Pennyworth, the villains were apparently able to shoot Batman with a wooden arrow during a confrontation in the Batcave, but after Batman was presumed dead, Two-Face turned on Gordon and Alfred, explaining that this was what the coin-toss meant (With this in mind, it suggests that Two-Face always planned an alliance as the only solution to the vampire Batman and the coin-toss just determined whether he would honor the alliance afterwards or betray his 'allies' immediately).
Injustice: Gods Among Us
In a world where the Joker's actions led to the destruction of Metropolis and Lois Lane's death at Superman's hands, Two-Face is drawn into the resulting 'debate' over Superman's more violent, 'pro-active' approach to crime, taking over a Gotham news station, only for Superman to vaporize his coin with heat vision before Dent can decide which news anchor he will kill.
Behind the Scenes
- Some viewers have stated that in The Dark Knight, Two-Face wasn't dependent on the coin to make his decisions, and if the coin dictated that he couldn't kill his target, then he'd find a way around that. For example, when Two-Face met Sal Maroni in his car and the coin dictated that Maroni couldn't be shot, Two-Face shot his driver instead, and caused Maroni to possibly die in the following car crash. Two-Face may have already thought ahead, however, and ensured that Maroni would live but would be severely injured and possibly be unable to inform anyone of his actions.
- While previous versions of Two-Face's Coin were based on silver dollars, the coin that was used in The Dark Knight was a replica of an actual 1922 Liberty Head Silver Dollar. That was in keeping with the theme of realism in the film series.