It makes a lot more sense if you know about chains. A chain is 22 yards, and there are 80 chains in a mile. There are also rods (a quarter of a chain) and furlongs (10 chains)

So: 3 Barleycorn in an inch 4 inches in a hand 3 hands in a foot 3 feet in a yard 5.5 yards in a rod 4 rods in a chain 10 chains in a furlong 8 furlongs in a mile

... And of course there's the overlapping systems of length for manufacturing, agriculture, maritime, and horse racing, which have their own, separate subdivisions and largest units, but usually you can get away with just the nail, the fathom, the nautical mile, and the span.