What is a rotation of 90 degrees about the origin?

You could rotate it in either direction, like a steering wheel.

-- If you rotate the whole plane 90 degrees to the left, then the old x-coordinate

of every point becomes its new y-coordinate, and its old y-coordinate becomes its

negative x-coordinate.

-- If you rotate the whole plane 90 degrees to the right, then the old x-coordinate

of every point becomes its new negative y-coordinate, and its old y-coordinate

becomes its new x-coordinate.