viginti

Viginti is indeclinable, like every Latin cardinal from quattuor up to centum. The form is an old dual underneath, "two tens", which is why it is not built transparently on duo and decem the way triginta is built on tres.

Counting past it gives viginti unus or unus et viginti, both correct. The numbers just below the round tens go backwards instead: undeviginti for nineteen, undetriginta for twenty-nine, literally "one from twenty", "one from thirty". The ordinal is vicesimus, the distributive viceni, the adverb vicies. The corpus spelling uiginti is nothing but the manuscript habit of writing u for v. French vingt and Spanish veinte are the direct descendants.