Zwanzig, vierzig, fünfzig – but: dreißig
It says zwanzig, vierzig, fünfzig.... but not dreizig, but dreißig. Isn't that rather inconsistent? What is the reason for this?
The question is quickly answered. The reason for this "inconsistency" with the tens lies far back in German language history, because it can be explained with the so-called High German or second sound shift in the early Middle Ages. We would like to quote here the "Wörterbuch der sprachlichen Zweifelsfälle" from the Dudenverlag (Mannheim 2016), which explains the reasons very clearly:
"The reason why one writes dreißig (thirty) with ß, but the other tens with z (zwanzig, neunzig/twenty, ninety) is the following: the tens are formed by appending the suffix -zig, which goes back to a Germanic word beginning with t (cf. got. tigus = 'decade, ten number'). In the composites of basic numbers as first member and this word as second member, the initial t usually occurred behind a consonant (zwan-, vier-, fünf- etc.) and was then, according to the phonetic laws, shifted in the High German sound shift (6th-8th century AD) to a closing sound with following fricative ([ts] = z).
Between vowels this did not occur; here t was shifted to a fricative, a sharp s ([s] = ß), hence: three-ß-ig." And so this exception has survived until today.