Valid and Invalid Moves

The rule states that a player cannot make suicide moves. Lets look at Fig 1, Position 1. Black cannot play at any of the points marked 'a' or 'b' since this would either take away the last liberty of his own stones (at points 'a') or place a stone so that it has no liberties at all (at point 'b'). Note, however, that Black can play at 'a' in Position 2 even though his stone will not have any liberties by itself. This is ok because this stone will be connected to the other black stones, which as a group (connections and groups explained here) have some liberties.

Position 1	Position 2	Position 3

Fig 1. Valid and invalid moves - the rule of "no suicide".

There is one exception to the "no suicide" rule - and it is illustrated in Position 3 of Fig 1. Now it is allowed for Black to play his stone at 'a' because even if it takes the last liberty of his own stones, at the same time it also takes away the last liberty of the marked White stones! Black can also play at 'b', although this specific situation, called Ko, has its own rule which guides the players and explains what can and cannot be done in such position.