What Is the Threefold Repetition Rule?
The threefold repetition rule applies when the same position occurs three times during a game. It is not enough for the same pieces to be sitting on the same squares, though. The same player must be to move, and the position must have the same available possibilities, including things such as castling and en passant rights.
The rule usually appears when neither side can find a useful way to change the position.
A common example is perpetual check. Imagine a queen repeatedly checking the opposing king. The defending king positionings itself away, a check is delivered by the queen and the king revisits the square it was already on. If this keeps happening, the game may eventually qualify for threefold repetition.
The important point is that the draw is not necessarily automatic under this rule. A player normally has to claim it.
How to Claim a Draw by Repetition
Over the board, a player can claim a draw when the position has occurred three times, provided the requirements of the rule have been met.
The claim can be made before making the move that creates the third occurrence, or after making that move, following the procedure required by the competition rules.
Online chess handles things a little differently. Chess platforms can keep track of the positions automatically, so you do not have to remember whether the same position has appeared twice already. Depending on the platform and situation, the draw may be offered or declared automatically.
This is worth remembering if you mainly play online: the computer is keeping track of details that can be surprisingly difficult to follow during a real game.
Why Threefold Repetition Happens
Perpetual check is probably the easiest example to picture, but it is not the only reason threefold repetition occurs.
Sometimes both players simply run out of useful options. A rook may move back and forth along a file, for example, because neither side wants to give up ground. In other cases, repetition is a deliberate defensive resource. A player who is worse may keep checking or threatening the same thing until the opponent has no realistic way to avoid repeating the position.
It can also happen when players are happy with a draw and see no reason to take unnecessary risks.
Threefold repetition should not be confused with fivefold repetition. Under the latter rule, the game is drawn automatically when the same position occurs five times.
Conclusion
Threefold repetition is essentially a way of recognizing that the same position keeps coming back without either player making progress.
Practically, you should learn to notice when positions are repeating themselves, especially when there are numerous checks or you're defending a difficult position. The threefold repetition can save you from a loss, gain a draw or stop you from wasting your time attempting to force something that isn't there.