Yellow Submarine

Free, one-sided, and fixed polyknights
There are three common ways of distinguishing polyominoes and polyknights
free polyknights are distinct when none is a rigid transformation ( translation, rotation, reflection or glide reflection ) of another ( pieces that can be picked up and flipped over ).  one-sided polyknights are distinct when none is a translation or rotation of another  ( pieces that cannot be flipped over ).
fixed polyknights are distinct when none is a translation of another  ( pieces that can be neither flipped nor rotated ).  Yellow submarine shows polyknights of various types with 2 cells.