Visual Guide: How to Write and Spell Numbers 21 Through 30 in English Correctly

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The modern hyphen acts as a binding agent in English grammar, but its deployment across numerals follows strict, inflexible guidelines. According to standard reference systems, including The Chicago Manual of Style and the AP Stylebook, writers must insert a hyphen in every compound cardinal number from 21 (twenty-one) through 99 (ninety-nine).

Leaving the hyphen out creates non-standard English. Writing "twenty one" as two distinct words violates standard editorial style, while running them together as *"twentyone" produces an outright spelling error. The rule holds steady regardless of whether the phrase operates as a noun, an adjective, or a measurement indicator.

The rule ceases the moment you land on thirty. Because thirty represents a single, monolithic root decade, it stands alone with no hyphen. However, the exact moment you advance to 31, the compound rule snaps back into place: thirty-one, thirty-two, and onward.