Lozenge.

How it goes together

Build the Boat castle: king 7h, bishop 8h, the right gold on 5h, the 6i gold and the 7i silver still at home. Then the left silver steps up from 7i to 6h, beside the king, and the right silver steps up from 4h to 5g, in front of the 5h gold. The 5g silver is guarded twice, by the 5h gold behind it and the 6h silver diagonally behind it. The 6i gold guards the 6h silver from below, and the king is the only piece that guards that gold. 菱 (hishi) is a lozenge, a diamond shape.

A Lozenge is a Boat with both silvers moved up: the first seven moves below are the Boat.

Move by move

Black only, from the start position: the Boat castle’s seven moves, then the two silvers, the left one first, in the order en.wikipedia gives after Hosking.

Nine moves. Because the left silver went first, either silver could have made the last move, and Japanese sources name the shape after the one that did: the ▲５七銀右 form, the right silver on 5g.

What it is good against

More than the plain Boat (en.wikipedia), and neither silver is idle: the 5g silver is guarded twice and the left silver now stands beside the king. ja.wikipedia notes that, unlike the Boat with the left silver on 5g, this shape can grow into either a rapid attack or a slower plan of taking space with pawns.

Where it breaks

It keeps the Boat family’s weak points. ja.wikipedia warns that in every form of the Boat the 6i gold is held by the king alone, so a rook dropped on the bottom rank is dangerous, and that 8g, above the king’s left, is the weak square of the whole family.

When to build it

A static rook against a ranging rook. ja.wikipedia names its rapid attacks: the Right Silver-46 attack (４六銀右戦法) against a Fourth File Rook that keeps its silver on 3b, and a rook swing to 3h against a Silver Horns Central Rook. The same base serves for the Pawn-45 Early Fight and the Ponpon Knight (ポンポン桂).