It comes from Chuvash language. İnş forms counting numbers, and language researches think they used to count in tens, as they didn't need precise numbers. yedi

It comes from Chuvash language. İnş forms counting numbers, and language researches think they used to count in tens, as they didn't need precise numbers. yedi/seven (m) inş -> yētiminş -> yetmiş/seventy altı/six (m) inş -> altıminş -> altmış/sixty. see: https://www.nisanyansozluk.com/?k=altm%C4%B1%C5%9F https://www.nisanyansozluk.com/?k=altm%C4%B1%C5%9F (in Turkish) 90 is tokuz-on (dokuz-on, nine-ten), 80 is formed that way too. 40, 30, 20 have specific names in old Turkish. Which means they used those numbers a lot, but no evidence why. No connection to 4, 3, 2 whatsoever (old Turkish for 4 ist tört, for example). 50 (elli) comes from the same root as the word which means hand (el). You can perhaps guess why. Long story short: You just need to memorize them if you are speaking Modern Turkish because "historical reasons". Turks started using numbers a bit too early and early weirdness was never fully "corrected" :) What I like in Turkish is that you don't use the unnecessary "one" before hundred/thousand. 100 -> yüz, 200 -> iki yüz. you don't say "bir yüz" for 100, just "yüz".