How do I determine my sock size?

LaCrosse Men's Socks:

Medium: Sizes 6 - 8.5
Large: Sizes 9 - 12
X-Large: Sizes 12.5 - 15

LaCrosse Women's Socks:

Small: Sizes 4 - 6.5
Medium: Sizes 7 - 9.5
Large: Sizes 10 - 12.5