What is the difference between against-the-rule and with-the-rule astigmatism?

Astigmatism is when your eye’s cornea or lens is irregularly shaped. Normally, the cornea and lens are curved equally in all directions.

If we view the eye as a sphere, “against-the-rule” is astigmatism where the steepest curve lies near the 180-degree meridian (imaginary line connecting east and west points of the cornea), and “with-the-rule” astigmatism (line connecting north and south points) is near the 90-degree meridian. With-the-rule is by far the most common type of astigmatism.