In centuries past, merchants selling goods by number often supplied more than the nominal total. The baker’s dozen of 13 is well known. Less so is the measure of 120, which was once known variously as the great hundred, long hundred or small gross.
This number 120 is the result of measuring items in a mixed system of twelves and tens, in part a survival of the duodecimal system used by many civilisations in antiquity and of which relics like the dozen and the gross still survive. It’s known, though very rarely, as the tolfraedic system. In origin the word is Icelandic, from tólf, twelve, plus ráða, to speak. Relatives of the term were used in Iceland and throughout Scandinavia to distinguish between hundreds that were ten tens and hundreds that were ten twelves (in Icelandic called tolfrátt hundrað).
The long hundred was so widely used at one time in England that a proverb was created to remind people that:
Five score’s a hundred of men, money and pins; six score’s a hundred of all other things.
Quoted in Curiosities in Proverbs, by Dwight Edwards Marvin, 1916.
It was common, as one example out of many, to sell nails by this measure (though why pins weren’t is a curiosity now lost in history). The medieval Anglo-Saxon Chronicle even stated that the year is 305 days long. This wasn’t an astronomical error, but the tolfraedic system in action. Three long hundreds is 360; add in the extra five and you have the usual year length.