Counterclockwise Rotations

• 90° : $(x, y) \rightarrow (-y, x)$
• 180° : $(x, y) \rightarrow (-x, -y)$
• 270° : $(x, y) \rightarrow (y, -x)$

Clockwise Rotations

• 90° : $(x, y) \rightarrow (y, -x)$
• 180° : $(x, y) \rightarrow (-x, -y)$
• 270° : $(x, y) \rightarrow (-y, x)$

(Either one of these lists is easily derived from the other.)