In indexing Belgian marriage records I have encountered months that are written:
10 9bre 1866. How does one translate such a date?
7bre means september (septembre)
8bre means october (octobre)
9bre means november (novembre)
10bre means december (decembre)
all coming from the french language and because the Roman calendar began in March. Just add 2 for the nowadays calendar.
They originally come from Latin, actually, but what Lars said is otherwise correct: the numeral-based abbreviations are for the numeral-based month names, which are based on the new year starting in March. (The 25th, I think it was? Not March 1st, anyway.)
Thank you, Lars and Julia, for the very helpful replies to my question.
Atually the Roman Calendar started on the first signs of the new crescent moon of March, or the so called Kalends of March and therefore every year diiferently. It only knew 10 months and 304 days.
The abbreviations, as used here, are French through, not Latin. The 3 ending letters are from the French names of the months: September = Semptembre = semptembris. Nowadays Belgium officially has 3 languages: French, Flemish and German.