Underpromotion in chess is where a pawn gets “promoted” because it has reached the other end of the board, but a lesser piece than a Queen is chosen. In practice, this rarely occurs because you usually want your pawn to get Queened, as it’s the most valuable piece. Legally, you can choose a Rook, Knight or Bishop. Since normal pawn promotion is called “Queening”, should we call these Rooking, Knighting and Bishoping?
All of the rules of “pawn promotion” apply to underpromotion moves. The pawn move or pawn capture to the 8th must be a legal move. The new piece can move backwards, and has the normal rules for that piece. You are not restricted to the normal piece counts: you can have 3 or more Rooks, Knights or Bishops, and the Bishops can be on the same color squares as other bishops.
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