Pins: Frozen Pieces

A PIN happens when a piece can't move (or really shouldn't) because something more valuable stands behind it on the same line.

If the piece behind is the KING, it's an ABSOLUTE pin — moving the pinned piece would be illegal. If it's the queen or a rook, it's a relative pin: legal, but usually a terrible idea. Only bishops, rooks, and queens can pin — they're the long-range pieces.