Single Comment

The game Edge Chess, by Mitch Martin, might be added to this discussion. In that game, a piece that can move to a (single) square is also allowed to 'expand' and occupy surrounding spaces as well. In this way, any piece on any move can become what appears to be a twofold or fourfold piece. An enlarged piece is vulnerable (since it can be captured on any of the squares it occupies) and maneuverable, having the ability to use any of its occupied squares as a departure space. Effectively, the piece shrinks to normal size, then moves, then has the option of expanding again, which is an alternative behavior to the two- and fourfold pieces described in this article. As a result of the size-shifting, Bishops are not colorbound, and Pawns can move unusually quickly across the board by always expanding into the next square forward.

Also, the large variant Microorganism Chess by Mark E Hedden and Bryan Weaver contains an Amoeba piece (of which each player has two). To quote:

They move either as a bishop or jump 3 spaces like a rook. But, it has one unique characteristic. It can expand from being a piece that takes up 1 square to a piece that takes up two squares orthoganally next to each other. However, both of these squares must be empty in order for it to be able to expand. Once expanded, it can take two pieces in the same turn, and generally be very dangerous. Also, it can, once expanded, contract back to its original size.

I presume that the entire width-two path of the expanded Amoeba has to be unoccupied when it wants to move as a bishop. So, the Amoeba in its expanded form is an example of a true twofold piece.