The punctuation mark that joins words in compound modifiers such as “well-known author” is the hyphen.
A compound modifier combines two or more words to act together before a noun. The hyphen signals that “well” and “known” form one descriptive unit, helping readers avoid misreading the phrase. Similar examples include “high-speed train,” “last-minute decision,” and “three-year-old child.”
Hyphenation depends on position and meaning. A temporary compound modifier is often hyphenated before a noun—“a well-known author”—but may not be hyphenated after it: “The author is well known.” Permanent compounds listed in dictionaries, such as “cost-effective,” can retain their hyphens in both positions.
The hyphen is not the same as the en dash or em dash. It is shorter and primarily joins words, parts of words, or certain number expressions. Adverbs ending in “-ly,” such as “poorly written report,” commonly do not require a hyphen, although style guides differ in some edge cases.