The legend is that a foot is actually based of the length of King Henry I's foot. "Imagine two neighbouring farmers, one buying a field from another, one with huge feet, one with tiny. Each measures the field in feet, but one says the footage is less because he’s got a bigger footprint, and therefore he argues a lower price. It’s a recipe for a mighty mediaeval punchup. So Henry proffered his own royal foot, it’s said. Thus, a foot became 12 inches."