xiii. Directly a pawn reaches its eighth square it must be exchanged

for a piece.

[It is usual to change the pawn for a Queen, but it may be replaced by a Rook, Bishop, or Knight, without reference to the pieces already on the board. In practice it would be changed for a Queen or a Knight, seeing that the Queen's moves include those of the Rook and Bishop. Thus you may have two or more Queens, three or more Rooks, Bishops, or Knights on the board at the end of the game.]