A difficult two-mover

This is from my book "White to Play" by Alain White. First, note the set play. If BQ plays along diagonal, you have Qxh7 mate, and if QxQ Sf2 mate. Other black pawns are pinned or blocked, and if the bishop moves you have Qb1 mating.

So that is a block. Can you preserve the waiting nature of the problem with any quiet move? Try as you might, you cannot.

So please move the mouse over, or click on the 1.? to see the solution. After the key, white threatens Rg4 mating. We now have the changed mates or added mates, exd4 Qd5 mate, Kxd4 Qb4 is cute, original mates Qb1, and Qxh7 are still in play after the other two captures on d4 to prevent the threat.

The above kind of problem is called a block-threat. Very deceptive and very hard to solve, but with a sharp intake of breath or "aha" feeling when you solve it.

Sam Loyd, puzzle king

The first thing you do in a "mate in two" problem, is the examine the "set" play. This means you pretend it is Black to move first, and you find checkmate moves for White

Set Play:

1. ... d5 2. Sc5#

1. ... b2, Sb4/a3/e3/e1 2. Q(x)b4#

1. ... Ra3/b2 2. S(x)b2#

In this problem, every legal black move has a White mate provided in the initial position. Such a position is called a "block". Sometimes, to preserve the character of a block problem, you try to find a quiet White move, that keeps Black in this trouble. Can we do that here? Let us see. If Bd1 moves, Black can now play b2, and there is no pin on the knight(S) at c2, no mate Qb4. If Bxc2, Sxc2. Strong checking moves by the Queen are useless, since 1.Qa7ch? Kb5 and the king escapes. The White rook cannot move gainfully. It releases the e-pawn 1...e5+ by Black would not be nice. How do we find a quiet move?

Some more terminology for future use. Neither the Black p at b3 nor S at c2 are pinned, but they can be said to be half-pinned. A great English composer Comins Mansfield, made many problems where the main line of play uses halfpins. Entire books have been written about problems with half-pins "Het Half-pin thema" in Dutch.

Have you solved it yet?

Sam Loyd was one tricky guy. The key move does not preserve the quiet. It introduces a threat. Such problems with a block in the set play and a threat in the "key" (the solution's first move) are called "block-threat" problems and are both rare and difficult to solve. Here the key is 1.Sa3! (>2. Threat Qa7#) (By covering b5, the threat is set up for the mate.) Here are the variations

1. ... Kxa3 2. Qa5/a7#

1. ... b2, Sb4/xa3 2. Q(x)b4#

1. ... Rxa3 2. Sb2#