Protean Chess (Francfort): When a piece captures (King included), it takes the nature of the captured unit (without changing colour). A King capturing becomes a royal unit.

Anda: A non neutral piece (except a King) that gives a direct check becomes neutral. A neutral piece (except a King) that gives a direct check takes the colour of the side that moved it.

Neutral piece: Belongs to whichever side chooses to use it. It can therefore be moved or captured by White or Black, and in Circe it is reborn according to capture. A King may not be moved onto a square controlled by a neutral piece, because of self-check.

Royal piece: Piece that executes a function of the King on the board.