Trapezoid

The mouth or trapezoid is an interior template with a wide base. It is one of the most common interior templates after the bridge. The carrier of the template, i.e., the set of cells that must be empty for the template to be valid, is shown in the following diagram.

Common patterns

Gaining Ground

The mouth can be used to steal territory. For example, in the following situation

Red can play as follows:

Ladder Escapes

The mouth also allows some ladder escape constructions.

Dominated cells

If Blue wants to intrude into the trapezoid, the only possible intrusion point is the hex marked "*".

To see why, note that Red can play the following pairing strategy.

If Blue plays in one of the cells marked "a", then Red plays in the other one. Similarly, if Blue plays in one of the cells marked "b", Red plays in the other one. The end result is that all of Red's pieces are connected, and any Blue pieces other than in "*" are dead. Since Blue cannot achieve anything better than to get "*", playing at "*" immediately is at least as good as playing anywhere else in the template. In other words, from Blue's point of view, "*" dominates all other possible moves in the trapezoid's carrier.

The same holds from Red's point of view as well. If Red is to play anywhere in the trapezoid's carrier, the only sensible move is at "*", because this captures the rest of the carrier.