Revoke
When a suit is lead, each hand must follow suit if possible.
Not doing so is called a "revoke".
If the revoking side play a card to the next trick then the revoke is "established", and play continues. The revoke will be dealt with at the completion of play (see below).
Establishment of a revoke can be prevented. If the player who made the revoke realises they have revoked before their side play to the next trick, then they can speak up, call the Director, and they will be allowed to change their revoke card. If they are Declarer then there is no further issue, play continues. Cards played after the revoke card may be withdrawn, but this is at the choice of the non-offenders.
If they are a defender then the withdrawn card becomes a major penalty card and is left face up on the table. They must play this card at the first legal opportunity, and if their partner comes on lead then Declarer will be able to impose Lead Restrictions.
A player's partner can also prevent a revoke being established. If their partner doesn't follow suit then they can ask them if they are "having none ?", or you can say "no hearts partner ?" Defenders can ask each other, dummy can ask Declarer. (In fact defenders can ask Declarer, and Declarer can ask a defender, though there is little benefit to this.)
Established Revoke
When a revoke has been established, by that player's side playing a card to the next trick, the revoke cannot be undone. Play must continue until all tricks have been completed. (unless the revoke is on the 12th trick.) Play on, and call the Director at the end of play.
Now the Director will do an "adjustment" by transferring tricks to the other side, so the result of the play must be clearly agreed first. Depending on the circumstances of the revoke, the Director will transfer zero, one or two tricks to the non-offending side, and may even decide to award more tricks (damage).
The Director asks :
Did the revoking card win the trick ? It must be a trump, so we have to be in a trump contract.
Did the offending side win any subsequent tricks, tricks after the revoke ?
For each of these questions, if the answer is Yes, then one trick is transferred.
The Director will then announce how many tricks are to be transferred, and thus the new final result. The Director should then offer the non-offending side the opportunity to claim more tricks if they feel the revoke further damaged their play of the hand (e.g. Declarer was cut off from a long suit in dummy).
Disagreement
If the players disagree about the revoke, or the circumstances of the revoke, then the Director will ask them to quickly replay through all the tricks, arriving at the critical trick(s), and then examine the remaining cards.
This is important to resolve any dispute, so keep your cards in order.
Further thoughts :
If you revoke you are not obliged to admit it (Law 72B), either during play or at the end of play. However, you are not allowed to intentionally revoke, and you are not allowed to conceal a revoke.
There is no penalty for Declarer if they admit to a revoke before it is established. Declarer can never have a penalty card.
Dummy may draw attention to a revoke by the defenders, BUT only after the hand has been completed. So one of dummy's tasks is to stay alert for revokes !
Defenders may ask their partner if they have revoked. This can create unauthorised information, and can be a bit of a grey area. It is certainly illegal to use this as a secret message - e.g. "don't return my lead".
You are not meant to say "having none" when you show out of a suit.
If there is a revoke and it is not established, the non-offending side may withdraw any cards played subsequent to the revoke. They don't have to. If they do then the offending side may withdraw their following card only, which becomes a major penalty card for defenders. They don't have to withdraw the card. (see Law 62C)
If the revoker's partner wins the trick, this does not count as a one-trick transfer, but is counted as a subsequent trick. (see Law 64A)
Revoke on Trick Twelve (Law 62D) - if the revoke occurred on the 12th trick then we can correct the play as the offender had only one legal play. Cards exposed by a defender create unauthorised information to partner. Again, the non-offenders may withdraw any subsequent card played. If all four hands have been returned to the board then a revoke on the 12th trick is deemed as established and the Director will apply the automatic adjustment.
A revoke can be pointed out later in the session. BUT, if the non-offending side has made a call on the next board then the Director can only rule for damage. All four players would need to agree on a recreation of the played cards.
If the Director has called "Move" for the next round, we still make normal revoke rulings (Law 8B)
If you claim, then any revoke by your side has been established. A revoke on the current trick by the non-claiming side is NOT established. (Law 63A3)
The revoke adjustment used to be an automatic two tricks, this is no longer the case, since 2007.
The revoke adjustment can be zero tricks. There is not necessarily a penalty to the revoker, and you are not guaranteed a good result if the opposition revoke.
Advice :
If an opponent has revoked, or your partner, try very hard to be polite !
You may not touch the cards of another player.
You may not expose cards that you have played to a previous trick during the play. Once a revoke has been established you have to keep quiet, not showing or mentioning which cards were played by mistake.
It is considered bad form to hide a revoke by not showing your cards if the opposition make a claim. Many players will deliberately show their cards after a claim has been agreed, as a courtesy - "and I haven't revoked earlier".
If you think an opponent may have revoked earlier in a hand, you can ask to see all of their cards at the end of play. Or you may be able to look at the cards on the tablet.
If you think an opponent has revoked, don't claim ! Play out the hand to see all their cards.
The automatic revoke adjustment is usually a harsher penalty than having a major penalty card, so it is best to speak up before the revoke is established.
Keep your played cards in order at the end of play, don't rush to mix them up and return them to the board, just in case someone claims a revoke. The Director can try to recreate the play of the hand, but has the power to rule in favour of the side that hasn't mixed their cards (Law 66D). This is an area that can cause a lot of animosity between players.
If you have revoked, apologise to the opponents and partner, and then forget about it.
If you partner has revoked, just try "these things happen, don't worry, forget about it, on with the next hand ..."