The 1-1-1 Doubling Rule for Adding Suffixes

When a word has one syllable, one vowel, and one final consonant, double that final consonant before adding a suffix that starts with a vowel — that's why 'run' becomes 'running,' not 'runing.'

The rule in one sentence

If a word has one syllable, one vowel, and one final consonant (the "1-1-1" test), double that final consonant before adding a vowel suffix like -ing, -ed, -er, or -est. Run passes all three checks — one syllable, one vowel (u), one final consonant (n) — so it becomes running and runner.

It works on longer words too — if the last syllable is stressed

The 1-1-1 test can apply to just the final syllable of a longer word, but only if that syllable is stressed. "Begin" is stressed on the second syllable (be-GIN), so it doubles: beginning, beginner. "Visit" is stressed on the first syllable (VIS-it), so it doesn't double: visiting, visited.

Letters that never double

• W, x, and y are never doubled. "Fix" becomes fixed and fixing, not fixxed. "Snow" and "play" already have two vowels before their final consonant, so they wouldn't double anyway.
• Check all three conditions before doubling. Skipping straight to "does it end in a consonant" without checking syllables and vowels is the most common mistake.

Doubling shows up before -y too

• The -y suffix doubles the final consonant too. It's easy to think of -y as a separate case, but it follows the exact same rule as -ing or -er: sun becomes sunny, fun becomes funny, mud becomes muddy, and fog becomes foggy — all pass the 1-1-1 test, so the final consonant doubles before -y just like it would before -ing.
• The same checks still apply. Rain becomes rainy, not rainny, because two vowels (ai) sit before the n. Wind becomes windy, not winddy, because two consonants (n and d) already end the word — both fail the 1-1-1 test the same way they would before any other vowel suffix.
• This is why "sunny" and "muddy" aren't typos. Because -y starts like a vowel sound when it's added this way, English treats it exactly like -ing, -ed, -er, and -est for the purposes of the 1-1-1 rule — so if a word passes the three checks, it doubles no matter which of these five suffixes comes next.

This is one of two common suffix-adding rules — the other is what happens to silent-e words, covered in the drop-the-e rule. For the full picture of suffixes, see our suffix spelling rules guide.