Welsh person here! Base 20 counting is still fairly common when saying the date, eg  "yr ail ar bymtheg o Ionawr" = the 17th of january, but literally you're saying "the 2nd on 15th of January",(reply>), @Buddy Clem it's Cymru. And it's not that wales becomes cymru, it's that they're entirely separate languages, one isn't based on the other.,(reply>), My God! I have Welsh ancestry, but for the life of me, I can't read a word of it. It boggles my mind how WALES becomes CRYMU. Thanks to everyone for teaching me a little more about my heritage.,(reply>), See my comment above. Truth is, it's very irregular.<br><br>There is base 20 counting. But also "X on ten" and "X on five-ten" addition, "two nines" multiplication and "half hundred" division. It (mercifully) lacks subtraction, but formal Welsh counting is a very arithmetical affair.<br><br>And, as mentioned, irregular. There's "two nines" for 18 but, no, 16 is not "two eights", it's "one on five-ten". You could get away with switching "two nines" to "three on five-ten" - that's acceptable - but you can't switch 16 to "two eights", in the other direction.<br><br>It's arithmetical, but you can't just make up your own sums willy-nilly. There is an irregular pattern to it - with a few acceptable variations - that you've just got to learn.<br><br>Much like spelling in English. Which is so irregular - as English must be the least phonetic language around - that the best advice to give learners is not to even bother looking for patterns. Just treat the spelling of every word as irregular and learn their spellings individually as you learn new words.<br><br>Because though there are patterns in English spelling, of course, it's just such a minefield of exceptions that it's honestly far easier to do that, than try to keep a bunch of different overlapping patterns and all their exceptions in mind.<br><br>And, as mentioned, I say "formal Welsh counting" because there's also a modern colloquial variant - which is really what people use these days - which is nothing more than reading off the digits with column headings, e.g. 4329 = "four thousand three hundred two ten nine".,(reply>), 15 and 2 would be base 15 no?,(reply>), ugain, deugain, trigain, pedwar-ugain. Well done Tom for knowing that. Pob clod i ti boi.