Since most of the world’s number systems are decimal, prehistoric European numbers were also likely to have been decimal. However, evidence for this is scarce. Some insight comes from proto-languages, hypothetical early languages reconstructed from their modern descendants. Proto-Indo-European had a decimal system that counted into the hundreds. Some European number systems are vigesimal and may represent numbers developed before Indo-European-speaking peoples moved into Europe. One is the number system of Basque, a language spoken in northern Spain and southern France. Other vigesimal numbers are found in Celtic, Romance, and Germanic languages, as possibly spread by Viking and Saxon invaders. Duodecimal (base-12) counting is suggested by the “long hundred” and the “double gross” found in Germanic languages. Danish forms its odd decades as half-steps between even multiples of twenty. Breton and Welsh form their fifty as “half hundred,” and Bova Greek builds its higher decades on fifty. Such formations suggest that Europe was home to several non-decimal number systems before Indo-European speaking peoples brought in their decimal numbers.