To double or not to double that last consonant is something that differs between Australia/UK English usage and North American. I take this blog’s information from The Cambridge Guide to English Usage by Pam Peters of Macquarie University. The process creates new verbs from nouns and adjectives. Now, let me refresh you on consonants and vowels. Most of the letters in the alphabet are consonants, but in words we need vowels to make them readable. There are five vowels — A, E, I, O, U, but we can use Y as an honorary vowel at times. Now to the general rules of doubling a consonant:

Generally, you double the final consonant if 1. the vowel before the consonant is a single one (for example: regret becomes regretted), and 2. If the syllable before the suffix is stressed (eg. wetted), not unstressed (BUDgeted and MARKeted). The result for some sample words are as follows — skims, skimming; win, winner;  step, stepped; begin, beginner; bosom, bosomy; sequin, sequined; gallop, galloped.  Skims is not doubled because there is not a vowel following the consonant. The stress is on the first syllable in bosomy and again in sequined, which also has a combined vowel sound before the consonant. Likewise, the stress is on the first syllable in galloped. Now apply the same conditions to stir, stirred; deter, deterred; butter, buttered; knit, knitting; admit, admitting; audit, auditing. I will try to make it easier by showing the syllable stresses — deTERred, BUTTered, AUDiting.

English, of course, will make exceptions, and these include: 1. Words ending in “x” never double the final consonant (taxed, transfixed). 2. Words ending in “c” will “double” with a ‘k’ to preserve the “k” sound (panicked, trafficking). 3. A syllable that is identical with a monosyllabic word will double the final consonant (backLOG to backlogged, fellowSHIP to fellowshipped, handiCAP to handicapped and so on).

British English also doubles the consonant when a word ends in -l regardless of stresses in the syllables. However, American will apply the general rules and double only when the stress in on the last syllable. For example, in British English you would say revelled while American English applies the stress rule and spells it reveled.