Clues are the only form of explicit communication allowed in Hanabi. They are a resource in the game along with cards and strikes.
Following are the rules relating to clues for the Standard game.
In the Standard game, the team starts out with 8 shared clue tokens. Taking a clue action costs one clue token, taking a discard action awards one clue token, and playing a 5 awards one clue token. When the team has 8 clue tokens, this is considered the "maximum" allowed count: a player may not discard at this count, but they may play a 5, in which case a clue token is simply not awarded.
A clue action entails a player choosing a number or color, and telling one other player which of their cards match that characteristic (and therefore also which do not match it). For example, Alice may use a clue action to tell Bob which of his cards are red, or which of his cards are 5s. To further clarify, Alice must point out all of Bob's red cards when giving Bob a red clue.
One detail which has not yet been specified is whether Alice can tell Bob that none of his cards match a characteristic. The legality of so-called empty clues actually varies between rulebook localizations for the same Hanabi editions[1]. On hanab-live, the ability
It's unknown what's the reason for this variation; we speculate that it could be by translation error or by a change in mind between localized printings. ↩︎