CHESSDORRAMate in 1
Black to move
Solution
One solution line: a1=Q#.
Position analysis
What happens on the board after the solution.
The key idea
a1=Q# The king on c1 is in check from queen a1. Its own pieces occupy d1, c2. The other escape squares are covered: b1 — queen a1, b2 — queen c3, d2 — queen c3. No legal capture or block removes the check. It is checkmate.
How to find checkmate
Check the king’s escape squares and every available defense.
When check becomes checkmate
Checkmate occurs when the king is in check and its side has no legal move. With no legal move and no check, the result is stalemate.
Three ways to defend
First check whether the king can move to a safe square. Then look for a legal capture of the checking piece or a block. A knight check cannot be blocked. In double check, only a king move can get out of check.
Mate against every reply
For mate in two, choose a first move, list the opponent’s replies, and find a mate after each. The first move can be quiet. A quiet move leaving the opponent with no legal reply produces stalemate.
Sources
Check and king safety
How to identify check and test the legality of a reply.
King safety after a move
Check your own king after the move you are considering. A pawn can leave a file and expose its king to a rook check even if the king was safe before the move.
Pinned pieces
A pinned piece still attacks squares according to its movement rules. Count those attacks when testing a king move or castling.
Reading the line of check
Locate the checking piece first. If the check travels along a line, test whether it can be blocked. Then check captures and king moves.
Sources
Back-rank mate
How the king’s own pieces can trap it.
The checking line and escape squares
A rook or queen checks along the back rank. Look at the squares in front of the king: its own pieces may occupy them, or the opponent may control them. Then test captures and blocks.
Giving the king room
Moving a pawn in front of the king can create an escape square. Before pushing it, check whether the new square will be safe and which other squares the pawn move weakens.
Sources
Pawn promotion
Four choices on the last rank.
The four choices
A pawn that reaches the farthest rank immediately promotes to a queen, rook, bishop, or knight of its own colour. You can choose a piece even if you already have one on the board.
When a knight is needed
A knight attacks squares a queen on the same square does not. Compare all four promotions: one may give the required check while another allows a defense or even causes stalemate.
Sources
Sources · Lichess