Compounds are words formed from two (or more) words. For example, black + board = blackboard.
They can be spelled in three ways:
- as a single word: flowerpot
- as two words: flower pot
- with a hyphen: flower-pot
So which one should you choose? A linguist has devised a rule which she says works for 75% of words:
- Compound verbs, adjectives and adverbs – use a hyphen (blow-dry, world-famous, well-nigh)
- Compound nouns:
- 3+ syllables – use a space (bathing suit)
- 2 syllables:
- second part has 2 letters – use a hyphen (make-up)
- second part has 3+ letters – as a single word (coastline).
You can read more on her website.
My even simpler rule: if you’re not sure, write it as a single word: blowdry, worldfamous, wellnigh, bathingsuit and makeup. Even if it’s wrong, it looks cuttingedge.