Krieger, Wendy(accessed 2008-09-15),“Twelfty for Decimal-Users”, in(Please provide the title of the work)
Base 120 is the largest of the historically attested bases […] There are references to a long or twelftywise count vs a short or teenty-wise count in all of the early Germanic writings.
Perhaps from Old Englishhund-téontiᵹ (compare hund-seofontiᵹ, hund-eahtatiᵹ, hund-niᵹontiᵹ, hund-endleofantiᵹ, and hund-twelftiᵹ for modern seventy, eighty, ninety, and nonstandard eleventy, twelfty, reflecting the old Germanic hybrid base-ten and base-twelve numbering system); perhaps reconstructed based on eleventy etc. and -teen.