Back-Rank Mate is a checkmate pattern given with one Rook or Queen. It occurs after the player castles King-side and the defending Rook no longer protects the King leaving the King on the back rank by itself blocked by the three pawns in front. This allows a Rook or Queen to move to the back rank delivering checkmate assuming no other pieces are or can come in between your rook or Queen and the King.
You’ll see this checkmate a lot if you’re a beginner. It even happens in games between higher rated players. At the Grandmaster level, the player will resign before this would happen.
The Back-Rank checkmate occurs after castling the King. If the Rook next to the King moves away from the 1 or 8 file also without moving any of the three pawns in front of the King. The King can be checkmated with a Rook or Queen moving to the 1 or 8 file. Unless the opponent has a piece that can either move next to the King or capture the Rook or Queen that put the King in check, it’s game over.
In this example, White forced a check on Black’s King with a Rook. Even though this gives up White’s Rook by letting Black capture it, it results in a forced checkmate. White’s Queen backs up the Rook and is able to re-capture Black’s rook after taking White’s Rook. After the Queen moves to the Backrank, Blacks King has no where to go.
How To Prevent This
Preventing this is simple, yet is easily forgotten in games. Back Rank checkmates typically occur during the end game. If you find yourself getting closer and closer to the end of the game and you haven’t moved a pawn in front of your King yet, assuming you castled to protect your King, the sooner you move a pawn in front of your King, the sooner you can remove the possibility of losing to this checkmate.
Inversely, if it’s getting towards the end of the game and you see that your opponent has not yet moved a pawn in front of their King, you can try and plan your next moves to win the game with a back rank checkmate.