The Queen.
In this position two Queens, a Rook, a Bishop and the two Kings are on the board. The Black Queen a4 which can move like a Rook or like a Bishop has the following moves at its disposal: to b4, c4, d4, capture on e4, ai, a2, a3, a5, a6, a7, a8, b5, c6, d7, b3, capture of c2. The White Queen, however, has a very limited range, because it is pinned by the Rook e8. If the pin would be released it could go to h7 and there, supported by the Bishop c2, Checkmate the King; as it is, the Queen must either capture the Rook e8 or suffer capture by that piece.