The numbers ‘twenty’ to ‘ninety’ end in ‘-ty’.
Where does this part come from?

While it’s now a suffix, ‘-ty’ stems from a Proto-Germanic noun meaning “decade, a (group of) ten”.

For example, ‘forty’ comes from *fedwōr tegiwiz, literally “four tens”.

Only Icelandic, Swedish, and Elfdalian have preserved this word as a noun.

Click my new graphic to learn all about ‘-ty’ and its relatives:

@yvanspijk just a little typo, German vierzig
@yvanspijk and zig also exists in the meaning of umpteen in German
@monsieurbecker Thank you very much! I've corrected both things.
@yvanspijk , my usual Finno-Ugric comments: the IE ‘10’ has, of course, been borrowed into Permic (Komi das) and Hungarian (tíz), from Iranian, two separate borrowings.
Then there is this nice tradition: from the 19th century on, scholars have tried to explain the second part of Finnish yhdeksän ‘9’ and kahdeksan ‘8’ as borrowings from IE ‘10’. (The first syllables in the Finnic numerals and their cognates in Saami, Mordvin and Mari obviously represent the numerals ‘1’ and ‘2’, and the motivation of the whole numeral would be something like ‘1 [or 2] missing from 10’.) The most recent version as formulated by Koivulehto and Parpola was based on the idea that the Proto-Iranian -ts- didn’t have a direct equivalent in Finno-Ugric (which only had a palatalized *ć and a broad *č) and was therefore substituted with -ks-. However, the vowels don’t match. Holopainen (2019) rejects this etymology and finds Erkki Itkonen’s old idea more convincing (albeit not flawless): these numerals would derive from *ükti / *kakti e-k-sä ‘1 (or 2) does not exist’.
But of course, as almost always, there's a more recent Swedish loanword: tjog ‘score’ has been borrowed into Finnish as tiu ‘20’ (used only when counting eggs).
@johanna_laakso Thank you very much for these interesting facts from a language family I've barrely touched the surface of.
@yvanspijk Curious how in French, the number name pattern also changes after 70.
@yvanspijk I had never heard of Elfdalian and looked it up on Wikipedia. It was an interesting read, thanks for the hint that gave me the chance to learn something new. :D
@yvanspijk @secbox
Slightly related story you may find interesting: I grew up in Saarland, a resource-rich border area which association changed multiple times between France & Germany after several wars. People there speak German, with an accent close to Luxembourgish. My grandma used the word "dix", just like the French word for ten (but pronounced diggs, not diss as in French) to say "often". And "dixer" (diggser) for "more often".
Miles away from "oft" or "häufig" in German.
@musevg @secbox Ah, yes, that word is related to German 'dick' ("fat"). In the southern regional languages of the Netherlands, they use 'dik(s)' to say "often" as well. It comes from a medieval combination with 'wile' (now German 'Weil(e)'), meaning "many times".
@yvanspijk @secbox
Whoa, that's interesting! So it's not related to French dix but to southern Netherlands diks? Amazing!