Grimshaw

A Grimshaw is an interference theme in composed chess problems. It usually involves two long-range pieces of the same color, such as a rook and a bishop, whose lines of action cross on one square.

In one variation, the first piece moves to the intersection and, by occupying it, blocks the other piece's line. In another variation, the second piece uses the same intersection and blocks the first. This reciprocal obstruction is the core of the theme.

The interference matters because the opposing side can exploit whichever defensive line has been closed. A rook defense might prevent its own bishop from controlling a mating square, while a bishop defense might cut off its own rook from a different square. The point is not merely that two pieces collide, but that each creates a different weakness by obstructing the other.

Its closest relative is the Novotny. That theme also exploits the crossing of two lines, but an opposing piece is first placed on the intersection as a sacrifice, forcing one of the defenders to capture it.