bis

Bis is the numeral adverb for two and stands to duo as semel does to unus. The series runs semel, bis, ter, quater, quinquiens, and none of them declines. It is not the same as the distributive bini, "two each", though the two often turn up together.

As a prefix it is everywhere: biennium, "a two-year period", bipes, "two-footed", and bissextus, the doubled sixth day before the March calends that the Julian calendar inserted, which is why the Romance languages call a leap year bisiesto or bissextile. The proverb every Latin class meets is bis dat qui cito dat: he gives twice who gives quickly. English takes biscuit, "twice-cooked", by way of French.