ko threat

A move made in the anticipation of forcing a local reply so that a stone, currently invulnerable due to ko, can be recaptured. In determining what moves are valid ko threats, the proponent must visualise the sequence where his opponent ignores the threat and instead plays to end the ko, thus allowing the proponent a second follow up move local to the ko threat. The size of this local gain for the proponent has normally to be weighed against the size of the local gain that the opponent obtains from finishing the ko. Such a comparison does not have to be considered when the ko threat sequence is linked to the ko sequence - see internal ko threat.

In the diagram white 1 has just captured a single black stone at
A in an ongoing ko fight for the life of the black corner stones by disputing the second eye at B. This ko is highly favourable to White (see hanami ko ), but nonetheless C thru H are considered as candidates for ko threats. The internal ko threat at C is perhaps the best threat. It must be answered by a White atari play just below which leaves further ko threats for Black to come. The other internal ko threat at D would be answered by White at C ending further threats. Although not an internal ko threat, E threatens to recapture the lower corner for Black which is roughly equivalent to the size of the top left corner. The stone at E would be immediately captured but would leave a second ko threat at F. Playing at F or G is not recommended because although these are valid threats, after White's reply at E, no further local threat remains. The move at H, because it has little follow up, is at this stage rather too small, and thus liable to be ignored by White.