30/360
30/360 is a day count fraction - the oldest, most simple-minded and yet elegantly pragmatic of all day count fractions — which basically divides the interest payable in a year into 12 equal sums. Magically effective, even though no actual month of the year has 30 days, and no year has 360 days.
You might well ask why we don't call it 1/12, and indeed that is one of those many excellent questions to which the financial markets, mighty as they are, can supply no good answer.