Just starting the 9th vocab unit in the second, southern, course.
Curious as to why raising “two fingers” is “die vees” and not “die veeseth”?
I think in Welsh you do not have to make the noun a plural if there is a number involved.
Numbers in Welsh are quite tricky but I think basically, as a rule of thumb, numbers 1 to 10 use the singular noun (wyth bys) and higher decimal numbers use the plural noun (un deg pedwar bysedd) … but vigesimal teens are split and use the singular noun after the first part (pedwar bys ar ddeg). This is just the tip of the iceberg. 
That is all very helpful. Dolch.
Gruntius: higher decimal numbers use the plural noun (un deg pedwar bysedd)
Minor point, but you forgot that it’s 14 of fingers for higher numbers - undeg pedwar o fysedd - with the o softening the following word as it normally does.
That’s not a minor point, diolch Rob.