Now combine the ideas we have been examining in this chapter, and this time from Black's side: work with coordinated attacks from two angles, and use them against protection supplied to the mating square from above. Black has two pieces aimed at f1, and White has one piece guarding the square—but the guard is on f3 and thus is in the way of Black’s rook. It’s not a problem so long as Black starts with QxR+. This forces White to reply RxQ and then permits Black to finish with RxR#. But of course if Black starts with RxR the idea fails, because after the reply RxR the f1 square is attacked once and still defended once. Move order matters.

As an aside, ask yourself this: if Black blunders by starting with RxR, does it matter whether White replies with RxR or QxR? It might seem unimportant since the piece used is not in immediate jeopardy and either way White still has a guard trained on f1. But in fact it matters very much. The key to seeing why, as often is true with tactics, is to consider any checks in the picture—here, any checks that will be available to your opponent. After 1. …RxR; 2. RxR, Black has 2. …Rb1+. 3. Rf1 is no defense for White; he gets mated right away with RxR. Instead White blocks the check with 3. Be1, where his queen guards the bishop. But now Black has 3. …Qe2, adding pressure to the e1 bishop that is saving White’s game. White cannot avoid a material calamity.

Even without following out these complications, the preference for 2. QxR rather than RxR can be explained on general principle. RxR weakens White’s back rank, removing the only defender from it and thus creating a nasty vulnerability so long as White’s king is blocked in by his pawns. Put that together with the check Black is waiting to give on the b-file and you have plenty of reason to worry about 2. RxR. (Now you can see what a "weak" back rank means: it generally refers to situations where a king is stuck behind its pawns and has no rook or queen on the rank able to defend against pieces that land there.)