Hyphenated compound adjectives

A compound adjective is hyphenated if the hyphen helps the reader differentiate a compound adjective from two adjacent adjectives that each independently modify the noun. Compare the following examples:

"acetic acid solution": a bitter solution producing vinegar or acetic acid (acetic + acid + solution)
"acetic-acid solution": a solution of acetic acid
The hyphen is unneeded when capitalization or italicization makes grouping clear:

"old English scholar": an old person who is English and a scholar, or an old scholar who studies English
"Old English scholar": a scholar of Old English.
"De facto proceedings" (not "de-facto")
If, however, there is no risk of ambiguities, it may be written without a hyphen: Sunday morning walk.

Hyphenated compound adjectives may have been formed originally by an adjective preceding a noun:

"Round table" → "round-table discussion"
"Blue sky" → "blue-sky law"
"Red light" → "red-light district"
"Four wheels" → "four-wheel drive" (the singular, not the plural, is used)
Others may have originated with a verb preceding an adjective or adverb:

"Feel good" → "feel-good factor"
"Buy now, pay later" → "buy-now pay-later purchase"
Yet others are created with an original verb preceding a preposition.

"Stick on" → "stick-on label"
"Walk on" → "walk-on part"
"Stand by" → "stand-by fare"
"Roll on, roll off" → "roll-on roll-off ferry"
The following compound adjectives are always hyphenated when they are not written as one word:

An adjective preceding a noun to which -d or -ed has been added as a past-participle construction, used before a noun:
"loud-mouthed hooligan"
"middle-aged lady"
"rose-tinted glasses"
A noun, adjective, or adverb preceding a present participle:
"an awe-inspiring personality"
"a long-lasting affair"
"a far-reaching decision"
Numbers spelled out or as numerics:
"seven-year itch"
"five-sided polygon"
"20th-century poem"
"30-piece band"
"tenth-storey window"
A numeric with the affix -fold has a hyphen (15-fold), but when spelled out takes a solid construction (fifteenfold).
Numbers, spelled out or numeric, with added -odd: sixteen-odd, 70-odd.
Compound adjectives with high- or low-: "high-level discussion", "low-price markup".
Colours in compounds:
"a dark-blue sweater"
"a reddish-orange dress".
Fractions as modifiers are hyphenated: "five-eighths inches", but if numerator or denominator are already hyphenated, the fraction itself does not take a hyphen: "a thirty-three thousandth part".
Fractions used as nouns have no hyphens: "I ate only one third of the pie."
Comparatives and superlatives in compound adjectives also take hyphens:
"the highest-placed competitor"
"a shorter-term loan"
However, a construction with most is not hyphenated:
"the most respected member".
Compounds including two geographical modifiers:
"Afro-Cuban"
"African-American" (sometimes)
"Anglo-Asian"
But not
"Central American".
The following compound adjectives are not normally hyphenated:

Where there is no risk of ambiguity:
"a Sunday morning walk"
Left-hand components of a compound adjective that end in -ly that modify right-hand components that are past participles (ending in -ed):
"a hotly disputed subject"
"a greatly improved scheme"
"a distantly related celebrity"
Compound adjectives that include comparatives and superlatives with more, most, less or least:
"a more recent development"
"the most respected member"
"a less opportune moment"
"the least expected event"
Ordinarily hyphenated compounds with intensive adverbs in front of adjectives:
"very much admired classicist"
"really well accepted proposal"