A Grimshaw is a device found in chess problems in which two pieces arriving on a particular square mutually interfere with each other. It is named after the 19th-century problem composer Walter Grimshaw. The Grimshaw is one of the most common devices found in directmates. Examples with description The theme can be understood by reference to the displayed example by A. G. Corrias (published in Good Companion, 1917). A. G. Corrias example The problem is a mate in two (White must move first and checkmate Black in two moves against any defense). The key is 1.Qb1, which threatens 2.Qb7#. Black has three ways to defend against this. One is to play 1...c3, giving his king a new flight square at c4, but this unguards d3, allowing White to mate with 2.Qd3#. It is the other two black defenses, however, which show the Grimshaw theme. Black can play 1...Bb2, thus cutting off the white queen's path to b7. However, the bishop on b2 interferes with the a2 rook and stops it moving along the rank - this allows White to play 2.Qh1# (after a different black move, this would not be possible because of 2...Rg2, blocking the check). Black can instead play 1...Rb2, cutting off the white queen with the rook rather than the bishop. However, just as the bishop on b2 interferes with the rook, so the rook on b2 interferes with the bishop, allowing White to play 2.Qf5# (a mate not otherwise possible, because of 2...Be5, blocking the check). It is this mutual interference between two black pieces on the one square (in this case, a rook and a bishop on b2) that constitutes a Grimshaw. Second Example The key in the puzzle on the right is 1. Qd2. This move has no threat, but it leaves black in zugzwang: Black must either move one of his bishops or rooks, or move a pawn. However, any bishop or rook move must unguard one of the squares of d5, d6, d7 or d8, allowing White to mate on d5, d6 or d7 with the queen, and d8 with the knight. The lines are: 1. Qd2! 1... Bxd2 2. Nd8# 1... Rb7 2. Qd5# (2... Bxd5 not possible) 1... Rc6 2. Qd5# (2... Bxd5 not possible) 1... Bb6 2. Qd6# (2... Rxd6 not possible; 2... exd6 not possible due to pin) 1... Bc6 2. Qd6# (2... Rxd6 not possible; 2... exd6