Rod and Pole

A rod is indeed a unit of measurement, 16.5 feet or 5.5 yards. It is also known as a pole or—especially in the USA—a perch, so leading to that euphonious set of measurements that were printed on the back of every child’s exercise book when I was young: “rod, pole or perch”, which we used to delight in quoting, though none of us had come across any of them in the real world.
The rod was one of an important set of measures that were subdivisions of the standard mile. Four rods equal one chain (22 yards—still the length of a British cricket pitch between the stumps), 40 rods make one furlong and 320 rods equal one mile.
The name comes from the use of a rod as a measuring stick (quite a big one, you may agree ...). It’s first recorded in the fifteenth century; pole dates from about the same period.