The Scots rod was 6 ells of 3 Rhineland feet.
The German and Norse ruthen are nearly always either of 12 or of 16 feet.
How came it that the English rod was fixed, about the time of Edward I, at 5-1/2 yards = 16-1/2 feet?
There is reason to believe that it was originally 5 yards, at first in Roman feet, then in Rhineland feet.
A length of 5 yards and 1 or 2 inches (= 1/(8 × 40) of the Roman mile) survives in the Dorsetshire ‘goad’ or ‘lug.’[[19]]
The Cornish rod or yard is 2 staves of 3 yards = 6 yards. There was, as late as 1540, a rod of 6 yards, ‘every pole containing eighten footes of the kinges standard.’
The rod of Guernsey, of Lancashire and of Ireland is 7 yards; it is the French perche of 20 pieds = 21·36 feet taken roughly at 21 English feet; this, and the Cheshire rod of 8 yards = 4 fathoms, are probably of Norman origin.
The English rod of pre-Norman and early Norman times was probably the Teutonic rod of 16 feet, as seen in the Roll of Battel Abbey. How did it become 16-1/2 feet?
I cannot absolutely solve the question; I can only offer the possible hypotheses:
1. That 5-1/2 yards was a compromise between a Southern rod of 5 yards and a Northern of 6 yards. But the former length only survived in the Dorsetshire lug, probably from Roman times, and 16 feet is the probable length of the Southern rod. And such a compromise is most improbable. I know of no measure established as a mean of two different measures.