1. Only some letters are doubled: b, d, g, l, m, n, p, r, t.

2. We double the final consonant of a word before we add

-ed,     -er,       -est,      -ing,       -able       and       -y

to show that the vowel has a short sound.

2a. With a one-syllable word:

If the word ends with a Consonant + Vowel + Consonant, double the final consonant.

This called the C-V-C rule.

Examples:

hat – hatter
big – biggest
mum – mummy
run – running, runny
gut – gutted

2b. With multi-syllable words:

Double the final letter when the final syllable is stressed in speech.


Examples:

begin – beginning
prefer – preferring, preferred
transfer – transferred


NB:
In British English, cancel and travel are exceptions to this rule: (travelled, cancelled, cancelling are correct)

3. If the final syllable is not stressed, we do not double the final letter.

Examples:

listen – listening, listened
happen – happening, happened

Vowels: a, e, i, o, u
Consonents: all other letters