What Is Yozume? Why a Tsume Shogi Problem Has Only One Answer

Sometimes you mate the king in a tsume shogi book with a line that is not the one in the answer. Your line really does mate. So is the answer wrong, or have you missed something? This is where the word yozume (余詰) comes in. In English it is usually called a cook.

The Japanese Wikipedia article on tsume shogi calls the line the composer intended the sakui (作意), and calls any other mating line that works a yozume. A problem with a yozume anywhere except the final move is treated as unsound. I mentioned this briefly in The Finer Rules of Tsume Shogi. In this post I show, with positions, how a cook actually appears.

Squares are written the Japanese way: 1一 means file 1, rank 1, and rank 1 is the defender’s back rank. Moves marked ▲ are the attacker’s and △ the defender’s.

A 3-move problem with no cook

First, a problem with a single answer.

Position 1: defender’s king on 1一. Your gold on 3二. In hand: a rook.

The answer is ▲2一飛 △1二玉 ▲2二飛成, mate in 3. You drop the rook beside the king, where the gold protects it. When the king runs to 1二, the rook moves to 2二 and promotes to a dragon. A dragon also moves one square diagonally, so it covers the escape to 1三, and the gold protects it, so the king cannot take it.

Now look at other first moves. Dropping the rook one square further away, ▲3一飛, is also check. But after △1二玉, if you follow with ▲2一飛成, the king escapes to 1三. No mate. Of all the first moves, ▲2一飛 is the only one that mates in 3. The problem has one answer.

Add one pawn, and a cook appears

Position 2: Position 1 plus an attacking pawn on 1四.

The intended ▲2一飛 △1二玉 ▲2二飛成 still mates. But now ▲3一飛, which failed in Position 1, also works.

▲3一飛 △1二玉 ▲2一飛成

The pawn on 1四 covers 1三, so the king cannot go there. The dragon on 2一 gives check diagonally to the king on 1二 and controls the 2-file. The king cannot go to 1一 either. Mate. If the defender blocks on 2一 instead of playing △1二玉, ▲同飛成 (capturing on the same square and promoting) mates in the same way.

There are two first moves, and both mate in 3. That is a cook. If Position 2 were set as a 3-move problem, someone who answers ▲2一飛 and someone who answers ▲3一飛 have both mated the king. Even if the composer meant ▲2一飛, you cannot call ▲3一飛 wrong. That is why a problem like this is unsound.

In Position 1, the open square 1三 was what made ▲3一飛 fail. Closing 1三 with a pawn opened a second road. Add one piece that helps the attacker, and the ways to mate multiply. This is one way a cook comes about.

Cooks shorter than the intended line

A cook that mates in fewer moves than the intended line is sometimes called hayazume (早詰), an early mate. For example, a problem given as mate in 5 that can be mated in 3.

If you think you have found an early mate, the first thing to do is check the defender’s replies again. In tsume shogi the defender picks the reply that lasts longest. If in your line the defender always plays moves that get mated at once, there is usually a more stubborn reply. If it really does mate sooner, then the problem is unsound because of an early mate.

A different last move is fine

Look at Position 2 again. After the intended ▲2一飛 △1二玉, the last move can be ▲2二飛成, but ▲2二金 also mates. The pawn on 1四 covers 1三, so moving the gold to 2二 leaves the king nowhere to go. In Position 1, 1三 is open, so ▲2二金 does not mate.

When only the final move has two or more ways to mate, it is called saishūte yozume (最終手余詰), a final-move cook. According to Wikipedia, a final-move cook does not make a problem unsound. The last move is only a choice of which piece finishes an almost finished mate, and the thinking in the moves before it stays the same. Position 2 is unsound anyway, because it also has a cook on the first move.

Why insist on one answer?

In a real game, any mating line wins. Tsume shogi still fixes one answer, and there are reasons for that.

One is that the solver can check their answer. With two answers, when your line differs from the book, you cannot tell whether you went wrong or the problem has another solution. The confusion at the start of this post comes from exactly this.

The other is that it makes the point of the problem clear. In a sound problem, the idea the composer wants to show is packed into a single line. With a cook, you can mate without that idea, and the problem teaches less. In Position 2, someone who sees that ▲3一飛 works may never think about the line that drops on 2一 and promotes to a dragon.

Wikipedia also lists, among the rules of tsume shogi, that the best line must be uniquely determined. Having no cook is at the centre of that condition.

Solving problems without cooks

If you find a line in a book that differs from the answer, check the defender’s replies and possible blocks before doubting the answer. Usually there is a better defence somewhere in your line. Looking for it is good practice in itself.

Shogi365 is a tsume shogi app with 1,000 puzzles each of 3-, 5-, 7- and 9-move tsume. Every puzzle was made and checked in-house: the first move is unique, each attacking move along the line is unique, and there are no cooks. Any check can be played, right or wrong, and the defender replies with the most stubborn defence automatically, so you can see on the board where in your line the defender had a better move.

Basic tesuji are in Basic Tesuji for Tsume Shogi, and how illegal pawn-drop mates are handled is in Uchifuzume Explained.