Roots Ending in Consonant + Vowel + Consonant
If the root ends with a consonant plus a vowel plus a consonant, there are five possible situations when adding suffix that begins with a vowel.
1. If the last syllable is accented, then double the final consonant. (Of course, this applies to nearly any single syllable word.)
Example: stop, stopped
Incorrect: commit, commited, commitee
Correct: commit, committed, committee
2. Rule #1 does not apply to roots ending in x or w. The letters x and w are never doubled in standard English.
Example: tax, taxed; withdraw, withdrawing, withdrawal
3. Rule #1 does not apply uniformly to three-lettered words ending in s.
Example: gas, gases; but gassed, gassing
Example: bus, buses, busing, bused
(This is probably done with bus because there is also the English word buss which can be a noun or verb.)
4. When adding the suffix changes the accent so that the last syllable of the root is no longer accented, the final consonant is not doubled.
Example: permit, permitted, permitee
(The accent changes in permitee.)
Example: prefer, preferred, preference
Example: refer, referral, reference
5. If the root ends with a consonant plus a vowel plus a consonant, and the last syllable is not accented, simply add the suffix beginning with a vowel without making any changes to the root.
Example: marvel, marvelous, marveled (Accent on first syllable)
Example: deposit, deposited (Accent on second syllable)
Note: Traveler is normally spelled with one l, but Civil War General Robert E. Lee named his horse Traveller spelled with two l's.
This rule is typically American. British spelling often doubles the final consonant of the root, especially when the root ends with an -el.
British: marvellous, traveller, cancelled
American: marvelous, traveler, canceled
Roots not Ending in Consonant + Vowel + Consonant
If the suffix begins with a vowel and the root has other configurations, see the appropriate entry:
Adding Suffixes to Roots Ending in -y
Adding Suffixes to Roots Ending in Silent -e