What Is Aigoma? How the Defender Blocks a Check in Tsume Shogi

Aigoma (合駒) is a block. The Japanese encyclopedia Nipponica defines it as a piece dropped on the line of a check from a rook, bishop or lance to stop that check (Kotobank, in Japanese). In tsume shogi, the defender holds every piece that is not on the board or in your hand, so you have to assume any piece from a pawn to a rook can land there.

Tsume shogi answers contain moves that make you stop and ask “why a silver here?” Blocks are one of them. This article covers when blocks come up, which ones are illegal, and how the piece and the square change the result. The rule about futile blocks is in The Finer Rules of Tsume Shogi, so it is kept short here. The positions are small ones made for this article, and every line has been checked.

You only consider a block when there is an empty square in between

A block goes between the checking piece and the king, so it needs an empty square. These checks cannot be blocked:

• A check given right next to the king
• A knight check, since the knight jumps and there is nothing in between
• A double check, where you have to move the king, as covered in Discovered Check and Double Check

On the other hand, a check from a distance by a rook, bishop, lance, dragon or horse can always be met by a block. This is where the reading grows.

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

Drop the rook with ▲1五飛. The squares in between are 1二, 1三 and 1四. There are seven kinds of piece: pawn, lance, knight, silver, gold, bishop and rook. So there are 21 blocks by drop alone. Add △2二玉 and the △1二金 described next, and the defender has 23 replies. Most of them can be discarded at a glance, but that is how many there are.

Moved blocks

△1二金 in Position 1 does not drop anything. It moves the gold from 2一 to 1二, into the line. This is called a moved block (移動合, idō-ai).

It looks the same as a gold dropped on 1二, but it isn’t. A moved block leaves the defender’s hand untouched, and it empties 2一. That may give the king a new escape square, or it may remove a guard from a square that gold was protecting. When the defender has pieces on the board, look at whether they can step into the line too.

Blocks the defender cannot play

Any piece can land there, with some exceptions.

Only pieces that are not on the board or in your hand. If you hold both rooks, for example, there is no rook block.

Two pawns on a file is illegal for the defender too. A pawn cannot be dropped as a block on a file that already has a defender’s pawn.

Pieces with nowhere to go cannot be dropped. The defender cannot drop a pawn or lance on rank 九, or a knight on rank 八 or 九.

If two pawns rules out the pawn block, the defender has that many fewer replies. When the position has a defender’s pawn, check that file too.

The piece changes the result

The hard part of blocks is that the same square can give different results depending on the piece.

Position 2: defender’s king on 1一. Your pawn on 3三 and bishop on 4四. In hand: a gold.

The first move is ▲3二歩成, pawn to 3二, promoting. The pawn steps aside, so the bishop on 4四 reaches the king on 1一 through 3三 and 2二. This is a discovered check. The promoted pawn covers 2一 and 2二.

The defender has 15 replies: △1二玉, seven blocks on 2二 and seven blocks on 3三. After △1二玉, ▲2二角成 (the bishop promotes on 2二) is mate.

Now the blocks on 3三.

Pawn, lance, knight, gold or rook on 3三. ▲2二金, a gold drop, is mate in every case. The promoted pawn protects the gold, so the king cannot take it, and the piece on 3三 does not reach 2二.

Silver or bishop on 3三. Now ▲2二金 does not mate. A defender’s silver reaches diagonally backward, from its own point of view, so it can take on 2二. A bishop also moves diagonally and can take too.

A gold cannot step diagonally backward. A silver can. If you only think “a gold is stronger than a pawn”, you miss this. When reading blocks, what counts is not only the strength of the piece but also what it reaches from that square.

Against the silver block you take it with ▲同角成, the bishop capturing and promoting. Then △1二玉 is met by ▲2二馬, and a block on 2二 by ▲同と. Five moves in all. In Position 2, the silver and bishop blocks on 3三 are the replies that last longest.

Blocking away from the king

Look at Position 2 again. The same silver, dropped right next to the king with △2二銀, is taken by ▲同と, and that is mate. Dropped one square further away with △3三銀, it lasts longer.

A block placed away from the king is called nakaai (中合い). The Japanese Wikipedia article on tsume shogi says a nakaai is a block placed at a distance from the king, and that in tsume shogi it is only accepted if it substantially changes the line that follows.

A block closer to the king is not always stronger. In Position 2 it was the reverse. The silver next to the king was just captured. The silver further away guarded 2二. When a nakaai appears in an answer, there are two things to look at: which squares that piece reaches, and where your piece has to move to capture it.

How this differs from a futile block

A block like △2二銀 in Position 2, which is only captured and does not change the line, is a futile block. Book answers don’t count it, so it doesn’t appear in them.

That means a block written in an answer has a purpose. There is a reason it has to be that piece, or a reason it has to be that square. When you read an answer, that reason is what to look for.

An order for reading

A check from a distance can be read in this order.

1. Count the squares in between. If there is only one, you only need to look there.
2. Try the square next to the king first. As in Position 2 of The Finer Rules of Tsume Shogi, if your capturing piece has no protection and the king takes it back, the check fails right there.
3. Remember the move you wanted to play next. In Position 2 here, that is ▲2二金. Look for a piece that stops it. Asking “what reaches 2二?” finds it faster than going through all seven kinds.
4. Check your hand after the capture. A captured block becomes yours. A pawn comes with the limits of two pawns and the pawn-drop mate rule, while a gold can give mate right in front of the king.

In mate in 7 and mate in 9, the reading of blocks grows too. How to read them is in Reading 7-Move and 9-Move Tsume Shogi.

Reading blocks in Shogi365

Shogi365 contains tsume shogi from mate in 3 to mate in 9, with 1,000 problems for each length, 4,000 in total. The defender can use every piece not on the board as a block, and the screen shows the defender’s hand.

You don’t look up the answer. You solve by playing on the board yourself. Any check, right or wrong, can be played, and the defender replies with the defence that lasts longest. A check from a distance really does get met by a block you did not expect, so you find out on the spot that your reading fell short.

The move count is the actual count including blocks, and there are no problems with a futile block that you would simply recapture. After solving, Review lets you step back and forward one move at a time, so you can look at why that particular block was played.

All problems are free to solve. The app shows ads, and a one-time Remove Ads purchase turns them off. How to use the app is in Getting Started with Shogi365.