Fork
Also called Double attack, Knight fork.
One piece attacks two or more at once, so only one can be saved.
Knight to c7 checks the king on e8 and attacks the rook on a8. The king must move, and the rook falls.
Knights fork most often, because their jump is hard to see coming, but every piece, even a pawn, can fork. When forking the king, the check takes away the choice.
A fork is the commonest double attack. A double attack is any one move that makes two threats, including a discovered attack.
New to chess? The examples here use it. Play your first moves (five minutes), then learn chess from the beginning, one short lesson at a time.
Practise it here: ten puzzles on this idea, at a level you choose, counted in your progress.
Want more? Puzzles on this from real games, on Lichess, free and with no account. They are not counted in your progress here.
Learn and practise it
- Lesson: The fork