The -TT - in Latin Quattuor

Numerals share with pronouns the honor of being the best criteria of linguistic relationship; and yet among the names of the digits we find more exceptions to the principle of regular phonetic development, probably, than anywhere else in an equal group of unquestioned cognates. The exceptions arise, in the main, from the fact that the words are in use so closely associated with one another that they affect one another's forms.