Illegal drops 禁じ手

Three kinds of drop are forbidden. On this site an illegal drop is simply refused: the board won't let you make it.

Nifu — two pawns on one file 二歩

You may not drop a pawn onto a file that already holds one of your own unpromoted pawns.

Only unpromoted pawns count: a tokin on the file doesn't stop you.

And nifu is only about drops — moving pawns never creates two on a file anyway, since pawns only move straight ahead.

Uchifuzume — mate by a dropped pawn 打ち歩詰め

You may not drop a pawn that gives immediate checkmate. Here a pawn on a8 would be mate — Gote's king could neither capture it (the gold on b7 protects it) nor escape (the gold covers b8, the silver b9) — so the drop is illegal.

The rule is narrow. Every other way of giving the same mate is legal: dropping a different piece, as above, or moving a pawn up the board.

And a dropped pawn may give check, as long as it isn't mate:

Dead-end drops 行き所のない駒

A piece may not be dropped where it could never move: a pawn or lance on the last rank, a knight on the last two. It's the same rule as forced promotion, and a dropped piece can't promote.

Try it