The hyphen (-) is a punctuation mark used to join words and to separate syllables of a single word. Its main purpose is to eliminate ambiguity. Words connected by a hyphen are called hyphenated words, while words that have merged into a single word without a space are called unhyphenated or closed compound words.

When to Use a Hyphen

1. Compound Adjectives:

When two or more words work together as a single adjective before a noun, they should be hyphenated.

• She is a well-known author.
• He drives a state-of-the-art car.
• It was a last-minute decision.

However, if these words come after the noun, they are usually not hyphenated.

• The author is well known.
• His car is state of the art.

2. Numbers:

Use a hyphen to write out compound numbers from twenty-one to ninety-nine.

• He is twenty-five years old.
• There are ninety-nine problems.

3. Certain Prefixes:

Use a hyphen with the prefixes ex- (meaning former), self-, and all-. A hyphen is also used to avoid confusion with another word.

• His ex-wife is a lawyer.
• She has high self-esteem.
• The all-inclusive resort was amazing.
• I need to re-sign the contract (sign again), not resign from my job (quit).

Hyphenated vs. Unhyphenated (Closed) Words

Compound words can be written in three ways: open (two words, e.g., ice cream), hyphenated (e.g., mother-in-law), or closed/unhyphenated (one word, e.g., website). Over time, many hyphenated words become closed.

• Hyphenated: These words are joined by a hyphen and function as a single concept. Examples: well-being, state-of-the-art, long-term.
• Unhyphenated (Closed): These are words that have fused into a single word. Examples: website, notebook, bookkeeper. The word "bookkeeper" is famous for being the only unhyphenated English word with three consecutive double letters.

When in doubt, it's always best to check a modern dictionary, as usage can change over time.