To pass means not to make a move when it is one's turn to do so. It then becomes the other player's turn again.
The most common formulations of the rules of Hex do not envision passing as a possibility. For example, most online playing sites do not allow passing. Indeed, it is easy to see that passing is theoretically unnecessary in Hex: if passing were allowed, it is never to a player's advantage to pass. In other words, playing a piece (no matter which piece) always leaves the player in an equal or better position than passing would do.
Reasons for passing
However, there are several theoretical and practical uses for passing:
- Theory: many theoretical arguments about winning positions become easier to state and analyze if passing is allowed. For example, the well-known property that Hex (without the swap rule) is a first-player win is easiest to prove with passing: if the second player had a winning strategy, the first player could simply pass, thereby effectively becoming the second player. Since this would be a winning strategy for the first player, the second player cannot have had a winning strategy.
- Handicap: for unevenly matched players, the game is often more enjoyable if the more experienced player gives the less experienced player a handicap. One way of implementing a handicap is for the weaker player to be permitted two or more consecutive moves at the beginning of the game. In practice, the stronger player would do this by passing one or more times at the beginning of the game. However, since many online sites do not permit passing, playing with handicap is difficult on these sites. Players sometimes have to resort to playing deliberately weak moves instead of passing.
- Strength of win: sometimes a player's position is so strong that they can pull off a win even if the opponent gets two or more moves in a row. In that case, the player may choose to pass as a way of demonstrating the strength of their position to the opponent. In a friendly or teaching game, this can have a pedagogical value by illustrating a point. In a competitive game, it can be a way of making a win more impressive.
- Losing play: once a player realizes that they will lose the game, they would customarily resign. However, sometimes resigning is not possible or desirable for some reason. For example, some game sites do not allow resigning unless a certain percentage of the board has been filled. In that case, the losing player often resorts to playing nonsense moves to allow the winning player to complete a connection as quickly as possible. Passing would be more elegant than playing nonsense moves.
The ability to pass does not affect the outcome
Implementations of passing
- No online playing sites currently permit passing. However, this ought to be changed.
- The current version of the SGF file format for Hex does not provide for a passing move. It does, however, permit consecutive moves by the same player. It would be desirable to add an explicit passing move to the file format, as this would allow passing moves to be recorded and documented. For example, a comment attached to such a move might explain why the player passed (e.g., to give a handicap). This is not quite the same as simply omitting the move; for example, for some handicapping methods, after the first move, the second player can choose to swap (and then the first player will get two consecutive moves) or not to swap (and then the first player gets to move again). If there is no possibility of an explicit passing move, the second player must verbally announce "I am not going to swap", which is not elegant and will not be recorded in the game tree.