The 1-1-1 Rule

There is one "rule" in English spelling that holds 100% of the time! That is the 1-1-1 Rule. Here's what it says:

Words of one syllable (1) ending in a single consonant (1) immediately preceded by a single vowel (1) double the consonant before a suffixal vowel (-ing, -ed) but not before a suffixal consonant (-tion).

In other words:

1 syllable
1 vowel (lax/short)
1 consonant following the vowel
Hence: the 1-1-1 Rule

Words that have more than one consonant after the vowel don't double
Words with a tense (long) 'e' vowel drop the 'e' before adding '-ing' or '-ed' with no doubling of consonant

EXAMPLES

This rule also applies to multisyllabic words although with some exceptions:

Words ending in a single consonant (1) immediately preceded by a single vowel bearing primary stress (1) double the consonant before a suffixal vowel (-ing, -ed) but not before a suffixal consonant (-tion).

The following words in Canadian/British spelling double the consonant (they don't in US spelling); both spellings are acceptable although the Canadian/British version is the most frequently used. Notice, these words all end in 'l'.

If the final syllable does not bear primary stress, the final consonant isn't doubled.