Updated: Dec 2, 2020
In English, a truly beautiful person is a perfect 10 or someone dressed to the nines. But in France, the scale goes up to 31! The expression se mettre sur son trente-et-un - to get dressed up - and its variant être sur son trente-et-un - to be dressed up - use this unusual number to indicate elegant or special clothing, and by analogy, getting ready for a big occasion. Se mettre sur is an old way of saying mettre sur soi, to put on yourself - equivalent to s'habiller in modern French. OK, but why 31?
There is no consensus on where that number, and the phrase itself, come from. There are, however, a number of propositions. Linguists favor the 2 following possible origins, with #1 being the most credible:
1- Back in the Middle-Ages, weavers (tisserands) were known to cut back on the amount of thread they used to produce cloth. Rather than selling the inferior product at a lower price, they would try to offer it for the same price as a good quality item. The French government then passed a law that regulated the amount of thread used in each type of cloth. The highest quality contained 30 x 100 threads and became known as trentain. Being the most sophisticated and expensive kind of cloth one could find, trentain was worn by wealthy people only. It was mostly unknown to the rest of the population who would soon distort the name from trentain to trente-et-un (31.) Over time the original meaning of “trentain” was forgotten and replaced by “trente et un”.
2- In Prussia, soldiers received the visit of their military hierarchy every "31 of the month". This only happened seven times a year. For the occasion, the soldiers had to clean their barracks from top to bottom and dress in their best uniforms. After the inspection, they would receive a kind of maintenance supplement in order to better finish the month. The expression would have been brought back by French soldiers who had fought in Prussia during the Napoleonic wars.
Ils se sont mis sur leur 31 et sont allés à la fête.
They got dressed to the nines and went to the party.
Il est toujours sur son 31, quelque soit l'occasion.
He is always dressed up no matter what.