Underpromotion: Why You Sometimes Promote to Something Other Than a Queen

When a pawn reaches the last rank, make a queen. When you first learn chess, it is easy to never consider anything else. The queen is the strongest piece, so there seems to be no reason to choose a weaker one.

Mate puzzles, though, include positions where the right answer is not a queen. This post covers underpromotion, promoting to a rook, bishop or knight, and when it is the correct choice. Each position was checked with a program.

The rule

Promotion is covered in Article 3.7 of the FIDE Laws of Chess. When a pawn reaches the last rank, it must be exchanged, as part of the same move, for a new queen, rook, bishop or knight of the same colour. The choice is not limited to pieces that have already been captured, and the new piece takes effect immediately.

The pawn cannot stay a pawn. Which of the four you pick is entirely up to you.

English Wikipedia’s Promotion (chess) says that promoting to a knight or rook is rare in practical play, and promoting to a bishop is rarer still. Most of the time a queen is fine. But because underpromotion is rare, it is easy to miss when it matters.

Why a queen is usually right

A queen moves like a rook and like a bishop. Anything a rook or bishop can do, a queen can do too.

So the only reason to pick a rook or bishop is when the queen’s extra reach gets in the way. The most common case is stalemate: the queen covers so much that the opponent has no legal move left.

The knight is different. A queen cannot make a knight’s jump. There are checks only a knight can give, so you choose a knight not because weaker is better, but because it moves differently.

Position 1: a rook to avoid stalemate

Position 1: White king c6, pawn c7. Black king a7. White to move.

What happens after c8=Q? From c8 the queen covers b8 and a8 along the eighth rank, and b7 and a6 on the diagonal. White’s king covers b6 and b7. The black king’s squares are a8, b8, b7, a6 and b6, and every one is covered. Yet the king is not in check. Black has no legal move, so it is stalemate, a draw.

After c8=R, the rook covers b8 and a8 but not the diagonal. The black king can go to a6. From there it is king and rook against king, which White wins. The method is in How to Checkmate with King and Rook.

Promoting to a bishop or knight doesn’t stalemate either. But a king and one bishop, or a king and one knight, cannot mate a lone king, so the game is drawn for lack of material. The draw rules are in Draws in Chess.

So of the four choices, only the rook wins. You take the weaker piece to remove the queen’s diagonal.

The best-known example of this idea is the Saavedra position, where promoting to a queen walks into a stalemate trap and promoting to a rook wins. I mentioned it in Stalemate Explained.

Position 2: a knight that mates

Position 2: White king a1, knight g6, pawns f5 and f7. Black king h7, bishop g8, pawns g7 and h6. White to move, mate in 1.

The answer is f8=N, mate. The new knight on f8 checks the king on h7.

Look at the black king’s squares. g8 is blocked by its own bishop, g7 and h6 by its own pawns. h8 is covered by the knight on g6. The king cannot take that knight because the f5 pawn defends it. A knight check cannot be blocked, and no black piece can take the knight on f8.

f8=Q gives no check at all. There is no straight line or diagonal from f8 to h7. Black still has legal moves, so it is not mate. Taking the bishop with fxg8=Q is check, but the king simply takes back.

f8=N is the only mate in one here. A check that only a knight can give turns straight into mate.

Position 3: a knight fork

Position 3: White king g1, pawn d7. Black king f7, queen b7. White to move.

After d8=Q, it is queen against queen with Black to move. Black can even start with Qb1+. White has gained nothing.

d8=N+ is check. The knight on d8 attacks both the king on f7 and the queen on b7. Black has to move the king, and then White plays Nxb7 and wins the queen. Even if the king steps to e7 or e8 to attack the knight, the knight leaves for b7 with the capture and is safe there.

A move where one piece attacks two is a fork. A knight fork is at its strongest when one of the targets is the king, because the opponent has to deal with the check first and has no time to save the other piece. Forks are covered in Basic Chess Tactics: Fork, Pin and Skewer.

When a bishop is right

A bishop is the rarest choice of the four. It is correct only when both a queen and a rook would stalemate, and a bishop leaves the opponent a move and still wins. Even the rook’s straight lines have to be in the way, and that combination doesn’t come up often.

Wikipedia describes bishop promotions as something you mainly see in composed problems and studies. In real games, one way is to go in this order: queen first, a rook if the queen stalemates, a bishop if the rook does too.

What to check before promoting

Before you play a promotion, check two things.

First, the opponent’s legal moves after a queen appears. Count every square the king can go to, and see whether any other piece or pawn can move. If nothing can move and there is no check, it is stalemate. Then see whether a rook solves it by removing the diagonal.

Second, what a knight would attack from the promotion square. Would it check the king? Would it hit the king and a big piece at once? A square where a queen gives no check can still be a check for a knight.

Compared with shogi

Shogi players have a close relative of this: choosing not to promote, called narazu. The reason is often the same. The stronger piece covers too much, or loses a move you need.

The mechanics are different, though. In shogi you either promote or stay as you are, and each piece has one fixed promoted form. In chess the pawn cannot stay a pawn, and you pick from four pieces. A shogi player declines promotion to keep a move the piece would lose, like a silver’s backward diagonals. A chess player chooses a knight to gain a move the pawn never had.

Why declining promotion is the answer in tsume shogi is in Not Promoting in Tsume Shogi, and how promotion works in shogi in general is in Shogi Promotion and Drops.

Promoting in Chess365

Chess365 is a checkmate puzzle app with 1,000 puzzles each of mate in 2, 3, 4 and 5. You always play White, you can make any legal move, and Black answers with the most stubborn defence.

When you move a pawn to the last rank, a “Promote to” panel opens with queen, rook, bishop and knight. The app never picks a queen for you. There aren’t many, but some puzzles are solved by promoting to something other than a queen. When the panel appears, run through the two checks above before you tap the queen.

If you leave Black with no legal move and no check, the app shows “Stalemate”, and the attempt fails. You have three retries. If you pick a queen in a shape like Position 1 and fail, try the rook next time, and the difference becomes very clear.

A hint lights up the piece to move and the square it goes to. You still make the move yourself, so on a promotion you decide which piece it becomes. After solving, “Review moves” lets you step back and forward one move at a time.

Every puzzle is free to solve, and a one-time Remove Ads purchase turns the ads off. How to read mate in 2 is in Chess Checkmate Puzzles for Beginners, and how to use the app is in Getting Started with Chess365.