Use a hyphen between the following types of compound modifiers:
Type of compound modifier
Examples
Number + noun
10-digit number
Adjective + noun
High-level description
Noun + noun
Parent-child hierarchy
Number + abbreviation
2-GB cartridge tape drive
Numbers spelled out from 21 to 99
Twenty-eight candles
Time
Three-thirty train
Adjective + participle
Open-ended question
Noun + adjective
User-centric design
Exception for common compound modifiers
Some modifiers are used as a single concept and do not require a hyphen for clarity. The following table provides examples of compound modifiers that do not need hyphens:
