H. G. Muller wrote on Tue, May 20, 2008 02:08 PM UTC:
What characterizes Chess variants:
1) move one piece at a time to an empty cell, or
2) capture an enemy piece by moving into its cell
3) win by capture of royal piece
4) many different piece types
5) a large fraction of the pieces are pawns
6) pawns are weak pieces which move irreversibly, and
promote to a stronger piece when advanced enough.
Some of these rules can be violated, but only if all other
characteristics are very close to a very common variant.