Useless triangle

When a piece's neighboring hexes are filled by the opponent such that that piece has only two empty neighboring hexes that are also adjacent to each other, then the piece is said to lie in a "useless triangle."

In the above diagram, the red piece at c5 and the empty hexes b6 and c6 form a useless triangle. The blue piece at e7 and the empty hexes e6 and f6 also form a useless triangle.

The important point is that unless the piece in a useless triangle is in that player's border row, the piece has effectively been removed from the game — that is, it cannot have any effect on the rest of the game regardless of the rest of the position.

See also the page Equivalent patterns.