A compound noun is one that is formed from a combination of two or more words. Most compound nouns are formed from a noun and another part of speech, but there are some compounds that can be formed from bases other than nouns:
- firetruck (noun + noun)
- blackbird (adjective + noun)
- downtown (preposition + noun)
- layaway (verb + preposition)
- roundabout (adjective + preposition)
Compounds which have been completely lexicalized are written without any space between them, as in the examples above. Some compounds are written with a hyphen (ex-wife), and compounds which aren't yet accepted as single words may be written as two words (ice cream). There are no hard and fast rules about which compounds are written which way.