Figure[Black to move]

Substituting the King to Create a Skewer.

Most of the techniques involved in creating skewers where elements are missing will be familiar from our work with pins, but the visual patterns look a little different. In this first study Black has a powerful resource in his doubled rooks but no immediate way to exploit it. Rather than obsess over the g-file, pause and obey the first law of tactical operations, viz., look at any checks you can give and their consequences. It doesn’t take long, as there are only two: Rg2+, which loses the rook to BxR, and RxB+, which loses the rook to KxR—but in a capture that requires the king to move, and thus requires an inspection of the king's new position. Survey the king’s new lines and anything else on them. Here White has put his king into alignment with his rook on the light-squared diagonal leading to d1. This calls for a skewer. Black plays Bg4+, and White has to move his king away; it can't guard the rook, so Black wins a piece with BxR.

The alignment of White’s king and rook after the preliminary exchange would be a visual cue to the skewer. You could have proceeded as well, of course, by simply looking for any checks Black can give on the board as it would look after White’s king moves, again seeing just two: Rg3+, which moves the White king unhelpfully, and Bg4+, which also moves the White king but this time allows BxR afterwards.

The large theme of the position is the idea of forcing a king to move by taking a piece that it protects—and then skewering it on its new square. Let's look at a couple of other applications of the same idea...