The subject of compounds, permanent or temporary combinations of words — temporary in the sense that they are “invented” for the occasion but are not in common usage — is a complicated matter because whether a compound employing a given word is open (with letter spaces between the constituent words), hyphenated, or closed (with no letter spaces) is usually inconsistent. Here’s a guide to the most common compound building blocks and how to treat compounds that use them.
1. Ache: Headache and similar words are always closed compounds.
2. All: Phrasal adjectives like “all out” are hyphenated before a noun (“This means all-out war”). In adverbial form, such phrases are open: “She went all out in an effort to impress him”).
3. Book: Many compounds that include book are closed (handbook); terms not listed in the dictionary should be open.
4. Borne: Some forms are closed (airborne); hyphenate compounds not found in the dictionary.
5. Century: This word is never hyphenated in a noun phrase (“twenty-first century”), but it is hyphenated to the rest of the phrase when the phrase serves as a phrasal adjective before a noun (“twenty-first-century mind-set”).
6. Counter: Compound nouns are almost always closed (countersign).
7. Cross: Leave compounds such as “cross section” open unless they are hyphenated or closed in the dictionary.
8. E: The prefix for electronic should be hyphenated in all uses, as well as lowercase except when the word begins a sentence. (I prefer email, ebook, etc., but this is a minority position popular in high-tech publications but not elsewhere.)
9. Elect: Hyphenate in such usage as “secretary-elect,” but omit hyphens when the other term is itself an open compound (“secretary general elect”).
10. Ever: Generally hyphenate before a noun (“ever-optimistic attitude”), but sometimes closed (evergreen).
11. Ex: Always hyphenate except with an open compound; in that case, use an en dash after ex. If your Web site does not use en dashes, reword to avoid awkward use of hyphens (“former vice president” rather than “ex-vice-president”).
12. Extra: Nouns and adjectives (extraterrestrial) are almost always closed, but check the dictionary; exceptions include “extra-point statistics.” Informal adverbs are open (“be extra careful”).
13. Foster: Compound nouns are always open (“foster family”); phrasal adjectives are hyphenated before a noun (“foster-care specialist”).
14. Free: Hyphenate before and after a noun when free is the second element in a compound (“scot-free”).
15. Full: Hyphenate before a noun (“full-time employee”).
16. General: Compounds with general as the second element are always open (“major general”); the first word, not general, takes the plural form (“secretaries general”).
17. Grand: Compounds denoting kinship that include this word are always closed (grandchild). Compounds in which grand is an adjective denoting status are always open (“grand dame,” “grand marshal”).
18. Great: This word is hyphenated in compounds denoting kinship (“great-grandchild”).
19. Half: Compound nouns are always open (“half dozen”). Phrasal adjectives are hyphenated before and after a noun (“half-eaten”). Check the dictionary for permanent closed compounds (halftime).
20. House: Compounds are open unless closed in the dictionary (“house cat,” but household).
21. In-law: Compounds with in and law as the second and third elements are always hyphenated; the first word, not law, takes the plural form (sisters-in-law).
22. Like: Hyphenate compounds in which like is the second element before and after a noun unless the compound is closed in the dictionary; hyphenate if the last letter of the preceding word is an l (snail-like) or a y (dormitory-like).
23. Near: Compound nouns are open (“near miss”); hyphenate adjectives before a noun (“near-term strategy”). Nearsighted, however, is closed.
24. Odd: Always hyphenate compounds including odd as the second element in the sense of “approximately” (thirty-odd).
25. Off: Phrasal adjectives are hyphenated before a noun (an “off-kilter column”); hyphenate adverbs (“sang off-key”). Hyphenate compounds in which off is the second element (bake-off). Note that style is occasionally inconsistent between on and off antonyms (on-screen, but offscreen).
26. Old: Compound nouns in which old is the last element are always hyphenated (“my six-year-old”); phrasal adjectives including the word are hyphenated before a noun (“six-month-old policy”).
27. On: Hyphenate adjectives and adverbs starting with on that are not in the dictionary. (See the note at off about inconsistency.)
28. Over: Compounds including this word, regardless of part of speech, are always closed.
29. Quasi: Hyphenate phrasal adjectives (“quasi-judicial proceedings”); some scientific terms are closed (quasiparticle).
30. Step: In kinship terms, always closed (stepmother) unless preceding grand or great (step-grandmother).
31. Style: Compound nouns are open (“art deco style”); hyphenate adjectival and adverbial forms (“family-style dining”; “dined family-style”).
32. Under: Compounds including this word, regardless of part of speech, are always closed.
33. Vice: Compound nouns are open (vice president), hyphenated (vice-consul), or closed (viceroy); hyphenate if not in the dictionary.
34. Web: Compound nouns are open (“web press” “food web”) or closed (webfoot, spiderweb); check the dictionary. Terms pertaining to the World Wide Web follow the same variations (“web page,” “webmaster”). Many publications style website as one word, and The Chicago Manual of Style has conceded this fact, but Merriam-Webster’s Collegiate Dictionary stands by “Web site” (which is also my preference).
35. Wide: When wide is the first element in the compound, hyphenate before a noun (“wide-ranging talents”). When it is the second element, hyphenate the compound if it does not appear not closed in the dictionary; many publications hyphenate such compounds in which the first element ends in y (community-wide).