Spelling: thinner or thiner? When to double a consonant before adding -er or -est

We add -er to a short adjective to form its comparative, and -est to form its superlative. We sometimes double the final letter of the adjective when we do this:

  • big ⇒ bigger ⇒ biggest,
    biger
    bigest

Sometimes, however, we don’t double the final letter:

  • cheap ⇒ cheaper ⇒ cheapest,
    cheapper
    cheappest

To understand the rule that can help us spell these words correctly, we first need to know what vowels and consonants are:

vowels = a  e  i  o  u;
consonants are all other letters (b  c  d  f  g, etc).

This is the rule:

Comparatives and superlatives:
when to double a consonant before adding -er and -est
When forming a comparative or superlative form, we double the final letter of a one-syllable adjective ending in consonant + vowel + consonant:

big ⇒ bigger ⇒ biggest,
fat ⇒ fatter ⇒ fattest,
thin ⇒ thinner ⇒ thinnest
We don't double the final letter when the adjective ends in -y or -w:

grey ⇒ greyer ⇒ greyest,
slow ⇒ slower ⇒ slowest
We don't double the final letter when the adjective ends in
vowel + vowel + consonant
or
vowel + consonant + consonant:

cheap ⇒ cheaper ⇒ cheapest,
old ⇒ older ⇒ oldest