**An updated copy of the Rules and Regulations is available here.
A word ending in a single consonant preceded by a single vowel generally doubles the final consonant before a suffix beginning with a vowel, if it is a word of one syllable or if the accent falls on the final syllable of the original word: plan, planning, planned; refer, referring, referred; bid, bidding, bidden; acquit, acquitting, acquitted; commit, committing, committed.
Exceptions: devil, devilish; benefit, benefited.
When a word ends in L, the final consonant is usually doubled before a suffix: cancel, cancellation.
NOTE: Words ending in a single consonant preceded by a single vowel do not double the final consonant before a suffix beginning with a vowel if a shift of accent results from addition of the suffix: prefer, preference; confer, conference.
Words ending in a silent E usually drop the E before a suffix beginning with a vowel; dine, dining; trouble, troubling, subdue, subduing; desire, desirable.
Exceptions: dyeing, singeing (to distinguish from dying and singing), shoeing, canoeing
Exceptions: argument, truly, ninth
Words ending in Y preceded by a consonant change Y to I before a suffix: heavy, heaviest; lively, livelihood; salary, salaried; necessary, necessarily.
Words ending in Y preceded by a vowel generally retain Y before a suffix: annoy, annoyance, annoying; journey, journeyed; gay, gayest.
Words ending in C with the hard sound of K – add K before I, Y or E; picnic, picnicking; panic, panicky; traffic, trafficking; mimic, mimicked.
Words ending in N retain the N before the suffix – ness; Sudden, suddenness; clean, cleanness; keen, keenness
An easy way to remember the rule for IE and EI is to learn the following rhyme:
I before E
Except after C
Or when sounded like A
As in neighbour and weigh
e.g. believe, reprieve, receive