Lesson
White to move. Main line: 1. Ra8#
Ra8 checks the king horizontally from the far side of the board. Distance does not weaken a rook: with no piece between a8 and h8, every square on the rank is attacked.
Black’s escape to h7 is covered by the bishop on c2 along the long diagonal, while g7 is controlled by the white king on f6. The king cannot run to g8 because the checking rook controls the entire eighth rank.
This position is useful for calculation discipline. Do not stop after noticing the rook check; name the defender of h7 and the defender of g7. Being able to account for each square prevents attractive checks that merely let the king escape.