I thought of these two words after learning recently that the singular form of one of them relates to Fannie Mae.

Problem:
One of these is not a proper word.

Explanation:
Fannie Mae — the U.S. Federal National Mortgage Association — has been in the news a lot recently, given the recent failures in the U.S. mortgage and banking industries.

I learned the other day that a “dwarf” in Fannie Mae lingo is the name given to a pool of mortgage-backed, Fannie Mae-issued securities with a maturity of 15 years.

As I wrote earlier this year, the plural form of the singular noun “roof” is “roofs” and never “rooves”.

In contrast, the plural form of the singular noun “dwarf” is either “dwarfs” — spelled D-W-A-R-F-S — or “dwarves” — spelled D-W-A-R-V-E-S.

This gives us the solution.

Solution:
“Dwarves” is a proper word (a plural form of the singular noun “dwarf”). “Rooves”, in contrast, is not a proper word.