Hi
Why is it correct to write a four generation family tree and not a four generations family tree? Is there a difference? (I'm not a native speaker sorry if it's a stupid question)
Thanks
Dana

It certainly seems to be a convention with many compound nouns that the second one remains in the singular form. Here are some more examples:

A twelve-storey building.
Two six-tier wedding cakes.
A four-layer lasagne.
A two-person carriage.
A six-chapter book.
Four ten-lap races.
The 24-month contract.
A ten-year lease.
One fifty-metre pool.
Numerous five-year jail sentences.
A two-horse race.
My four-generation family tree

I don't know the reason, but it may have something to do with the fact that if the main was plural, two adjacent plural nouns would sound very awkward: "two six-tiers cakes"; "several two-horses races".


PaulM