leeneia - I think the first measures were taken from different parts of the body. Reading your post I checked my right index finger; it is about 3'. 12' is 1"; the name foot tells where the measure was taken from.
And the length of a yard was determined by an English king who stretched out his arm and said: this is a yard. The rest is harmonizing. Before the definition of one weight and measure in England every community had them with the same name, but slight differences.
Here, too, the doudecimal system prevailed in earlier times.
The same it is with the old money systems. 12 pence to the shilling correspond very well with the 12 silver Pfennige to the Groschen in Charlemagne's realm.