Nana-Shogi is perhaps the smallest non-trivial chess variant in existence.
 
The following is based on Georg Dunkel's description of the game from his web pages.
Nana Shogi is played on a 3x3 board. Each player has three cube shaped pieces, one conventional king and two pieces with four values each.
One piece has the values: Rook, Chariot, Swallow's Wings and Go-between.
The other piece has the values: Bishop, Tile General, Cat's Sword and Dog.
These pieces are taken from Shogi (Rook & Bishop), Chu Shogi (Chariot, Go-Between), Dai Shogi (Cat's Sword), Maka-Dai-Dai Shogi (Tile General) Wa Shogi (Swallow's Wings) and Tenjiku Shogi (Dog).
 
At the commencement of the game the board is empty. Play begins with each player in turn (starting with black) dropping their king. Players may then choose at their turn either to drop a piece or to move a piece on the board.
Pieces (other than the King obviously) may be dropped in whichever value state the player chooses.
When a piece other than a King moves on the board, either with or without capturing, that piece's value changes. Values change sequentially; Rook to Chariot to Swallow's Wings to Go-between to Rook, etc and Bishop to Tile General to Cat's Sword to Dog to Bishop, etc.
It is illegal to drop any piece on the central square and it is illegal to check or to checkmate by dropping a piece.
 
Under George Dunkel's rules it is also illegal to checkmate when one has one or more pieces in hand. (This last rule was not implemented in the original zrf of Steve Evans.)