Fork

A fork is a double attack by a single piece. The knight is the classic forking piece because it attacks squares no other piece guards at the same time, and because its targets can never attack it back along the same line. But every piece forks: a pawn on a knight and a bishop, a queen giving check while eyeing a loose rook, a rook on the seventh hitting two pawns.

What makes a fork WORK is not the geometry, it is the targets. A fork on two defended pawns wins nothing; a fork on the king and a rook wins the rook, because the king must move. So the checklist is: which enemy pieces are undefended or more valuable than my attacker, and is there one square that hits two of them?

The strongest forks come with check. A check leaves the opponent exactly one move, and it is never the move that saves the other piece. When you see a loose piece, look for a check that also attacks it. When you see a king on an open file or diagonal, look for what else sits on that line.

The second half of the skill is seeing forks BEFORE they exist: the square is empty now, but a capture or a deflection will open it. The higher the level, the more the fork is the second move of a combination rather than the first.