Uchifuzume Explained: When a Pawn Drop Mate Is Illegal, and How Tsume Shogi Gets Around It

Article 10 of the Japan Shogi Association’s game rules (in Japanese) lists the fouls that lose the game on the spot. After two pawns on one file (nifu) and a piece left with no legal move, the third item is uchifuzume, the drop pawn mate. The rules define it as dropping a pawn from hand to give a check that cannot be answered.

If you solve tsume shogi, you have probably stopped at some point and wondered why you may not drop a pawn on a square that clearly mates. This post breaks the rule into its conditions, compares positions that are and are not fouls, and then looks at how tsume shogi problems get around it. The basic rules are in Tsume Shogi for Beginners, so here I stay with this one rule.

Positions are written in text. Squares use the Japanese notation, where 1一 is file 1, rank 1, and rank 1 is the defender’s back rank. Pieces without a label are the attacker’s (yours); the defender’s pieces are marked “defender’s”.

Three conditions together make it a foul

A drop pawn mate is a foul only when all three of these hold at once.

1. The move drops a pawn from hand. Moving a pawn already on the board does not count.
2. The pawn gives check.
3. The defender has no move that answers the check. In other words, that single move is mate.

If any one is missing, there is no foul. Giving check with a dropped pawn is fine, and so is mating a few moves after a pawn drop. Only one thing is banned: mating with the pawn drop itself.

A position where it is a foul

Position 1: Defender’s king on 1一. Your silver on 3二 and gold on 2三. In hand: a pawn.

A pawn dropped on 1二 gives check. The king cannot take it, because the gold on 2三 covers 1二. The silver on 3二 covers 2一 and the gold on 2三 covers 2二, so the king has nowhere to go. No defending piece can take the pawn either. There is no answer to the check, so the pawn drop on 1二 is uchifuzume, a foul.

With only a pawn in hand, this position has no mate in seven moves or fewer. The pawn would mate, and that is exactly the one move you may not play. This is the kind of moment where the rule matters.

With a lance in hand instead, the story changes. A lance dropped on 1二 gives the same check, but it is not a pawn, so it is simply mate.

Positions where it is not a foul

Mating by pushing a pawn already on the board.

Position 2: Defender’s king on 1一. Your silver on 3二, gold on 2三 and pawn on 1三. Nothing in hand.

The only difference from Position 1 is that the pawn starts on 1三 instead of in your hand. Push it to 1二 and the pieces cover the same squares as before, so it is mate. This one is legal. Mating by moving a pawn on the board is called tsukifuzume, the pawn push mate, and it is allowed.

A pawn check the king can escape.

Position 3: Defender’s king on 1一. Your gold on 2三. In hand: a pawn.

This is Position 1 without the silver. Drop the pawn on 1二 and it is check, but the king can step to 2一. There is an answer, so the drop is legal. Checking with a dropped pawn is no problem at all.

When a piece seems able to take the pawn but cannot

The third condition, “no move that answers the check”, needs care. A defending piece that seems able to take the pawn does not count if it is not free to move.

Position 4: Defender’s king on 1一, defender’s gold on 2一. Your rook on 4一 and gold on 2三. In hand: a pawn.

Drop the pawn on 1二. The defender’s gold on 2一 covers 1二, so it looks as if it can take the pawn. But if that gold moves, the rook on 4一 sees straight along the rank to the king on 1一. A move that exposes your own king to check is not allowed, so the gold cannot take. The king cannot take either, because the gold on 2三 covers 1二, and it also covers 2二. There is no answer, so this pawn drop is uchifuzume too.

Judging by sight, “there is a piece that can take it, so it is fine”, is where people go wrong here. Before dropping the pawn, check that the capturing piece can really move.

Why is it a foul?

Nobody knows for certain. The Japanese Wikipedia article on uchifuzume says a popular explanation is that the lowest foot soldier should not take the general’s head. The same article calls this a folk belief with no supporting sources, and points out that it cannot explain why a pawn push mate is allowed.

According to that article, the earliest written statement of the ban is in Ōhashi Sōko’s Shōgi Zushiki of 1636, which it describes as putting existing custom into writing. So whatever its origin, the rule has been in place at least since the early Edo period.

How tsume shogi gets around it

Tsume shogi composers use this rule on purpose. They show you a position where a pawn drop would mate, forbid that move, and make you look for another route. For the solver, the moment you notice “I cannot drop the pawn here” is often the way into the problem.

Position 5: Defender’s king on 1一. Your dragon (promoted rook) on 2三 and pawn on 3三. In hand: a bishop and a pawn. Mate in 3.

The first move that catches the eye is a pawn drop on 1二. The dragon on 2三 covers 1二, 2一 and 2二, so it would be mate. But that is uchifuzume.

The answer is a bishop drop on 2二. The dragon protects it, so the king cannot take it. The dragon also covers 1二, so the king’s only move is to 2一. Then push the pawn from 3三 to 3二 and promote it. The new tokin gives check to the king on 2一. The bishop on 2二 covers 1一, the dragon covers 1二, and the tokin covers 3一. The bishop on 2二 is protected by the dragon and the tokin, and the tokin on 3二 by the dragon, so neither can be taken. It is mate, by a pawn push.

The pawn in hand is never used in this problem. It is there to tempt you with a drop that looks like mate.

The Wikipedia article also sorts other ways around the rule into types. They come down to two ideas.

• Give the king an escape square. Move a bishop or rook without promoting so that it covers less, move a piece out of the way, or sacrifice a piece to open a square. With even one escape, the pawn drop is no longer mate, so it is no longer a foul. You check with the pawn, the king moves, and you mate it later.
• Let the defender take the dropped pawn. Bring a defending piece to a square where it can capture the pawn. With a capture available, the pawn drop is again not mate.

Both turn the pawn drop into a move that is not mate. Looking for a mate with a pawn already on the board, as in Position 5, is another answer to the same problem.

In a real game

In a real game, playing uchifuzume loses on the spot. Under Article 10, if the foul is found before the game ends, play goes back to the moment of the foul and the game is lost there. If it is found after the game ends, the player who committed it still loses.

From the defending side, the rule can work as a shield. If your opponent has only pawns in hand, a position where a pawn drop would mate you is one where that move cannot be played. When the opponent holds nothing but pawns in the endgame, it is worth checking whether your king is being saved by uchifuzume.

An app where the board enforces it

Shogi365 is a tsume shogi app with 1,000 puzzles each of mate in 3, 5, 7 and 9. You can play any check, right or wrong, and the defender replies automatically with the most stubborn defence.

You cannot drop a pawn that would be uchifuzume. When you select a pawn in hand, the squares where it would be a drop pawn mate get no marker, even with “Show legal moves” turned on, and tapping one does not place the pawn. It does not count as a failure either. A case like Position 4, where a piece seems able to take the pawn but cannot, is treated as uchifuzume too, and the drop is refused. Page 3 of the how-to-play screens, “The rules of the drop”, also explains that a pawn may not be dropped to give mate and that two pawns may never share a file.

Other finer conventions are in The Finer Rules of Tsume Shogi, and basic tesuji in Basic Tsume Shogi Tesuji.