Half-pin

In a pin, a unit cannot leave a line because doing so would expose its king to attack. A half-pin begins with a special arrangement: two Black units share the line between the Black king and a White rook, bishop, or queen.

At first, neither Black unit is fully pinned. If one moves off the line, the other remains as the king's sole screen. That remaining unit is now pinned because it cannot leave without exposing the king to check.

White exploits this immobility to give a mate that would fail if the pinned defender could move. In a complete presentation of the theme, each Black unit leaves the line in a different variation and pins its partner. The two units therefore exchange functions.

The White line piece is usually a rook, bishop, or queen. The types of the two Black units are less important than their shared placement between that line piece and their own king.