Long Distance

White has three queens on the board — an embarrassment of firepower, except all three are stuck on the a-file, the far-left edge of the board. Composer Florencio Mendes de Moraes built a forced mate in seven moves around exactly that constraint.

1. All three white queens can only ever have moved to squares on the a-file.
2. Black's king starts on g8 and has nowhere good to run.
3. Either of two first moves gets the mating net started — but only one choice at every step after that keeps the mate forced.

Find the seven-move sequence that forces checkmate.

Reveal the answer

1. Qa4 (or Qa3) Kg8 2. Qa1 Kf8 3. Q1a2 Ke8 4. Q2a3 Kd8 5. Q6a4 Kc8 6. Qa8+ Kc7 7. Q4a7#