Daily AncestorLatin ordinal numbers
Latin ordinal numbers are primus, secundus, tertius (first, second, third); in dates they usually end in -o, as in die decimo quinto, on the fifteenth day.
Ordinals say which one in a row: primus, secundus, tertius, quartus, quintus, and on to decimus (10th), vicesimus (20th, also spelled vigesimus) and millesimus (1000th). Dates use them in the ablative case, ending in -o: die vigesimo primo means on the twenty-first day.
Years are written the same way, so anno millesimo septingentesimo octogesimo means in the year 1780. Scribes shortened these long words heavily, writing millesimo as mill. with a small o.
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Daily Ancestor. "Latin ordinal numbers." Dictionary of Old Record Words. https://dailyancestor.com/dictionary/latin/latin-ordinals. Last checked 2026-10-10.