See CMoS 7.77–84, 5.91 | For compounds not covered here, text following British English spelling defaults to Collins; text following US English spelling defaults to Merriam-Webster.
See also Dashes; Money & Prices
Hyphenation should make the meaning of the text clear.
upper-middle-class neighbourhood
When phrasal adjectives (multiple words that combine to form a single modifier) precede a noun, they are hyphenated.
middle-class neighbourhood
If the phrasal adjective contains a compound noun, the entire phrasal adjective is hyphenated to clarify the relationship among the words (unless hyphens would cause ambiguity, in which case use an en dash).
tour-bus-filled resort town
army-surplus-style attire
If the phrasal adjective follows a verb, it is usually unhyphenated.
a well-trained athlete
an athlete who is well trained.
Do not hyphenate foreign phrases that form a compound unless the phrase is ordinarily hyphenated according to the relevant dictionary.
fin de siècle architecture (not fin-de-siècle architecture)
à la carte meal (not à-la-carte meal)
tête-à-tête approach
We do not use hyphens in compound modifiers that are proper nouns.
Queen Anne architecture
Use suspended hyphens where required for clarity.
two- and three-star hotels; 2-/4-bed dm
Use a hyphen when a unit of measure forms part of a compound adjective.
6m-high walls; six-year-old dog
Do not use hyphens between the number and abbreviated unit of measurement.
16oz mug of beer; a 2lb weight; 3.5km hike
Adverbs ending in -ly are almost never hyphenated.
a highly developed country