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