Cool Measurements

An inch is the length of the king's pinky between the knuckles.

      |<= 1 in. =>|
       ___________
      |           |
      |           |
      |    \ /    |
      |     ||    |
      |     ||    |
      |     | +__+
      |     |
      |     |
     ----/----

A handbreadth, or hand's breadth, is the width of the king's flat, closed hand (palm plus thumb width). There are two handbreadths in a span.
A span is the length between the king's pinky tip and thumb tip when his hand is spread out. There are two spans in a cubit.
A cubit is the length of the king's forearm (from elbow to middle finger). There are two cubits in a yard.
A foot is the length of the king's foot (duh!). There are three feet in a yard.
A yard is the length from the king's nose to the tip of his fingers (facing forward, arm spread out to the side).

Back in a time when there were no standards organizations or yardsticks lying around, this was a quick and easy way to measure things. It's still handy, pun disavowed, if you don't need to be exact (e.g., seeing if a piece of furniture will fit in a room). It can be tough to get the king to hold still while you use his hands and arms, of course.

The inch has been standardized, or perhaps it's the yard that's been standardized. The handbreadth has been standardized to the 4-inch hand. (A horse's height is measured from in hands from the ground to the withers, near the shoulder.) A foot has been standardized to 12 inches, and a yard has been standardized to 36 inches. A span is considered to be 9 inches and a cubit is considered to be 18 inches, although 16-inch cubits are also known. So a span is not 2 hands unless the span is 8 inches. The upshot is, the inch, hand, foot, and yard are modern standard measurements (in the UnitedStates at least), but spans and cubits, still being archaic, are up for grabs, so to speak.