To determine your face shape, evaluate the relationship between the length of your face, the width of your forehead and the width of your jaw line. For example, if your forehead and cheekbones are the two widest points of your face, and you have a narrower jaw line, you may have a heart-shaped face, like Reese Witherspoon.
Oval faces have foreheads only slightly wider than their chins, and the distances for the length and width of their faces are very close in measure, like Jessica Biel. Like Demi Moore's, square faces are angular, with the forehead width almost matching the width of the jaw line. Round faces have dimensions similar to those of square faces, but round faces typically have a softer chin and generally rounder cheeks, like Kirsten Dunst. Oblong faces are longer than they are wide, like Sarah Jessica Parker; sometimes they're called rectangular faces. Diamond faces, like Rihanna's, are similar to heart faces, with narrow chins and wider cheekbones, but diamond faces have narrow foreheads to match.
If you're having trouble visualizing, look in the mirror straight on, take a dry erase marker, and dot the widest parts of your forehead, cheekbones and jaw line on the mirror. Step away from the mirror, and then evaluate the relationship between the dots as above.