The Finer Rules of Tsume Shogi: Futile Interpositions, Leftover Pieces and the Shortest Mate

Sometimes you solve a tsume shogi problem, look at the answer, and cannot see why that line is the right one. Your own line also seems to mate. The defender in the answer did not pick the reply that gets mated fastest. An interposition would make the line longer, yet the answer does not interpose. Most of these puzzles have an explanation in the finer conventions of tsume shogi.

The basic rules (every attacking move is a check, the defender holds every remaining piece, a mating pawn drop is illegal, and so on) are in Tsume Shogi for Beginners. This post covers the conventions beyond that, the ones that decide how to read an answer. The definitions mostly follow the Japanese Wikipedia article on tsume shogi and the English one. The positions are my own, and I checked each of them.

I write squares 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.

Longest defence, shortest mate

In tsume shogi the defender avoids check with the best, which means the longest, line. The attacker, by convention, chooses the shortest of the mating lines.

Put together, the answer is “the shortest mate the attacker can force against the defender’s most stubborn defence”. When the defender’s reply in the answer differs from the one you expected, it is usually because the defender found a reply that lasts longer. And if, in your line, every defender reply gets mated at once, you may have missed the defender’s best move.

Futile interpositions: blocks that do not count

When you give check from a distance with a rook, bishop or lance, the defender can block by dropping a piece in between. That is an interposition (合駒, aigoma). A block that only gets captured and does not change the substance of the solution is called a futile interposition (無駄合い, muda-ai), and it is not counted in the move total.

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

The answer is ▲2二角, a bishop drop on 2二. The bishop gives check with the gold protecting it, the king cannot go to 1二 or 2一, and it cannot take the bishop. Mate in 1.

What if you drop the bishop far away instead, at 5五? The defender can interpose on 4四, 3三 or 2二. But every block is simply captured by the bishop, and after the capture the same check continues. None of these blocks has any effect on whether the king is mated. These are futile interpositions.

As I understand it, futile interpositions are not counted because otherwise the defender could stretch any line indefinitely. There is also the matter of pieces in hand. If you keep capturing blocks and then mate, the captured pieces stay in your hand. As the next sections explain, tsume shogi avoids that state, so a line full of futile blocks would make an unnatural answer.

Blocks that are not futile

A block is not always futile. If it stops the mate or changes the line, it is a real defence.

Position 2: defender’s king on 1一. Your gold on 3二. In hand: a rook.

The gold covers 2一 and 2二, so a rook drop from below at 1九 seems to keep the king off 1二. But the defender drops a block on 1二, right in front of the king. If you take it with the rook, nothing protects the rook, and the king takes it. This block on 1二 removes the mate, so it is not futile. Even if blocks on 1三 to 1八 would be futile, the single block on 1二 is enough to make the check fail.

The answer here is ▲2一飛, a rook drop on 2一. The gold protects it, so the king cannot take it and must go to 1二. Then ▲2二飛成, promoting the rook to a dragon on 2二, is mate, again with the gold’s protection. Mate in 3.

When you consider a check from a distance, I think it is worth first checking whether a block on the square next to the king works.

Leftover pieces

If the attacker still has pieces in hand when the king is mated, that is called 駒余り (koma-amari), leftover pieces. According to Japanese Wikipedia, early classical problems did not treat this as a fault, but today a problem whose intended main line leaves pieces over counts as unsound. Leftover pieces in a side variation are fine.

A related point is how the defender chooses between replies. The defender picks the longest line, and when several replies lead to the same length, the usual reading is that the defender picks the one that makes the attacker use up every piece in hand. The English Wikipedia puts it as the defender should exhaust the attacker’s pieces in hand.

For a solver, leftover pieces are a useful check. If your line mates but you still hold a piece, suspect that the defender had a better reply or that your first move is not the intended one. In a well-made problem, every piece in hand has a job.

Cooks and short mates

A mating line other than the intended one is called a cook (余詰, yozume). A cook shorter than the intended line is sometimes called 早詰 (hayazume), an early mate. A problem with a cook anywhere except the final move counts as unsound. An alternative only on the very last move does not make a problem unsound.

If you seem to have mated a mate in 5 in three moves, I think the first thing to do is look again at the defender’s replies. Usually the defender has a way to last longer. If it really does mate in three, then the problem has an early mate and is unsound.

Same-length and longer variations

A line where the defender plays something other than the main line is a variation (変化, henka).

Same-length variation (変同, hendō): a variation that mates in the same number of moves as the main line. If that variation leaves pieces in hand, the defender would not choose it, so it causes no trouble. If it does not leave pieces over, you can no longer say which line is the answer, and it counts as a flaw. A same-length variation in the last two moves is considered acceptable.

Longer variation (変長, henchō): a variation that takes more moves than the main line.

If you look at an answer and think “this other reply would last longer”, you may have found a longer variation.

How this differs from a real game

I see these conventions as the means of turning a composed problem into a puzzle with exactly one answer. In a real game, leftover pieces do not matter at all if you mate quickly, and blocks that look useless are a normal way to gain time. It is worth remembering that the conventions of tsume shogi are not the rules of a real game.

Also, a mating pawn drop or a repetition by the attacker counts as a failure in tsume shogi.

An app where the board keeps the rules

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. The defender can use every remaining piece to interpose.

Inside the app, an interposition and its capture count as real moves. In exchange, the app includes no puzzle whose main line contains a futile interposition that the attacker simply recaptures. So every puzzle mates in exactly the number of moves shown. In every puzzle the first move is unique, and none of the attacker’s pieces on the board or in hand, and none of the defender’s pieces, are superfluous.

Basic tesuji are in Basic Tsume Shogi Tesuji, and how tsume shogi helps in real games is in Does Tsume Shogi Make You Stronger?.