This is not a binding rule, and in English language tradition, it is sometimes recommended to spell out numbers up to and including either nine, ten or twelve, or even ninety-nine or

*one*hundred. Another system spells out all numbers written in*one*or*two*words (sixteen, twenty-seven, fifteen thousand, but 372 or 15,001).In German orthography, there used to be the widely followed (but unofficial) rule of spelling out numbers up to twelve (zwölf). The Duden (the German standard dictionary) mentions this rule as outdated. Twelve is a composite number, the smallest number with exactly six divisors, its divisors being*1*,*2*, 3, 4, 6 and 12.