A Grimshaw is a device found in chess problems in which two black pieces arriving on a particular square mutually interfere with each other. It is named after the 19th-century problem composerWalter Grimshaw. The Grimshaw is one of the most common devices found in directmates. The theme can be understood by reference to the displayed example by A. G. Corrias. The problem is a mate in two. 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 defences, 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#.
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#.
It is this mutual interference between two black pieces on the one square that constitutes a Grimshaw. The key in the puzzle on the right is 1. Qd2. If 1. Qb2? then 1... Bc7 defends. This move has no threat, but it leaves black in zugzwang: Black must either take the queen and allow 2. Nd8# or move one of his bishops or rooks. 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: By: Geometrist in , comment #50
The pieces involved in Grimshaws are usually rook and bishop, as in the previous example, although Grimshaws involving pawns are also seen, as in this mate in two example by Frank Janet : The key is 1.Qd7, threatening 2.Qf5#. As in the previous example, Black can defend by cutting White's queen off from its intended destination square, but two of these defences have fatal flaws in that they interfere with other pieces: 1...Be6 interferes with the pawn on e7, allowing 2.Qxc7# and 1...e6 interferes with the bishop, allowing 2.Qxa4#. It is this mutual interference between bishop and pawn on e6 which constitutes the pawn Grimshaw. There are several other non-thematic black defences in this problem — see below for them all.
Multiple Grimshaws
Sometimes, multiple Grimshaws can be combined in one problem. Here are two examples by Lev Ilych Loshinsky each with three Grimshaws: This was first published in L'Italia Scacchistica, 1930. It is a mate in two. The key is 1.Rb1, with the threat 2.d4#. Each of Black's defences produces a Grimshaw interference which stops him from capturing White's mating piece. Black's defences, with White's replies, are:
1...Re6 2.Nd7#
1...Be6 2.Bd6#
1...Rg4 2.Ne6#
1...Bg4 2.Bg1#
1...Rb2 2.Qxc3#
1...Bb2 2.Qf2#
There is one other black defence: 1...Rd6 leading to the simple recapture 2.Bxd6#. This second Loshinsky example, also a mate in two, is from Tijdschrift v.d. Nederlandse Schaakbond, 1930, and is one of the most famous of all chess problems. It is a complete block, and White's key, 1.Bb3, holds this block, making no threat, but putting Black in zugzwang. Black has six defences leading to three Grimshaws, one of them a pawn Grimshaw:
1...Rb7 2.Rc6#
1...Bb7 2.Re7#
1...Rg7 2.Qe5#
1...Bg7 2.Qxf7#
1...Bf6 2.Qg4#
1...f6 2.Qe4#
After other black moves, White can play one of the above moves to mate; the three exceptions are 1...f5, takingaway that square from the king and allowed 2.Qd6# and two recaptures: 1...Rxc7 2.Nxc7# and 1...Bxd4 2.Nxd4#.
A close relative of the Grimshaw is the Novotny, which is essentially a Grimshaw brought about by a white sacrifice on a square where it can be captured by two different black pieces – whichever black piece captures the white piece, it interferes with the other.
