Double-seki

Seki is a situation in go when two groups have locked each other in a stalemate. I.e., normally, by the end of the game, a group is either alive (because it has two eyes) or dead (because it does not have two eyes and is surrounded by another group). In seki, if the Black moves in a given part of the board, he loses his group. And the same goes for the White. So, neither of the players can move locally. Liberties surrounded by stones in seki are not counted (according to the Japanese rules).

This is an example of a double-seki (on the left top corner and on the left-middle):

(source)

As you can see, if the White moves within the seki (e.g., at D17), its group gets captured (by Black E15). The same goes for the Black (D17 leads to White B17). And the same is true for the other two groups (playing at C11 by either Black or White leads to an immediate capture).

In fact, it’s also true for the White’s "inside" group and the Black’s group around it (White can threaten to capture the Black’s "middle" group by playing at E15, but then it gets captured at D17). So, this can be called triple-seki!