cool feature of Brythonic languages is that they have an "inverse number" system

rather than marking nouns as plurals, classes of nouns have expected numbers, which can be negated using suffixes, for example Breton -enn, and Welsh -yn/-en e.g. in Breton, gwez, means trees, the word is innately plural, gwezenn = a single tree

no other indo european languages do this as far as i know, it exists in some native american (tanoan) languages too

more examples (all from Welsh) adar "birds", aderyn "birds" mefus "a bed of strawberries", mefusen "a strawberry" plant "children", plentyn "a child" and coed "forest", coeden "a tree"

aw just realised my mistake adar "birds", aderyn "a bird" *