Clearly we need to find a move that threatens mate. It turns out that this move is by the rather out-of-play Nc1, but it’s not immediately obvious that 1.Na2! does carry any threat. White is angling after possible mates by Nb4 and Nc3, but these threats are at present defended respectively by the bQ and the bR. Aha! But there is a threat, 2.c4+! This move, to the intersection of the lines h4-b4 and c7-c3, would present Black with an impossible dilemma. If he replied 2…Rxc4 then the R can be decoyed away: 3.Nb4+! Rxb4 (the point is that …Qxb4 is longer possible) and now another square the Rook had been guarding is available to White: 4.Qc6#. Something very similar would happen after 2…Qxc4: 3.Nc3+ Qxc3 4.Be4#.
But of course it is Black to play and he has two first moves that defeat the threat. One is 1…Qa4, pinning the cP. But something similar (but more spectacular) happens now: 2.Qc4+!. Now we have 2…Rxc4 3.Be4+! Rxe4 4.Nc3# (this wouldn’t have worked as a threat because after 2.c4+ Rxc4 3.Be4+ there’s 3…Qxe4)
and 2…Qxc4 3.Nc3+ Qxc3 4.Be4#.
Finally, Black can defend with 1…Nxc2, and this time (hurrah) it’s 2.Bc4+ that works: 2…Rxc4 3.Nb4+ Rxb4 4.Qc6#
and 2…Qxc4 3.Qc6+! Qxc6 4.Nb4#. There are other black first moves which fail uninterestingly.
When moves such as these are to the intersection of two orthogonal lines of guard the device or motif or theme, call it what you will, is described as a Plachutta interference. The better-known situation in which the move is to the intersection of an orthogonal line of guard and a diagonal line of guard is known as a Nowotny interference. These interferences do sometimes crop up in games, and there are probably other occasions on which a player has failed to spot such an opportunity.
Christopher holds the Grandmaster title for Chess Problem Composition and uses his skills to write a regular column for the Bristol Chess Times. He is also a longterm Horfield Chess Club player (where he is acting secretary).