Double-click on any word and see its definition from Cambridge Dictionaries Online.

English vowels can be pronounced differently, having both short and long sounds:

			Fat (short)
			Fate (long)
			Equate (short)
			Equal (long)
			Dinner (short)
			Diner (long)
			Hop (short)
			Hope (long)
			But (short)
			Butane (long)

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.

But how do you know when to double the consonant and when not to? There are a number of things to consider.

Firstly, we only double a consonant if it comes at the end of at word.

Slop - slopped - slopping BUT slope - sloped - sloping

Secondly, we only double a consonant if a word ends in one vowel followed by one consonant.

			Dig - digging
			Shut - shutting

BUT

			Fool - fooled - fooling
			Bend - bending

Thirdly, we need to consider words that end in one vowel followed by one consonant, but contain two syllables. We only double the consonant in these words if the last syllable is stressed:

Last syllable stressed

			Deter - deterred - deterring
			Unplug - unplugged - unplugging

Last syllable unstressed

			Happen - happened - happening
			Gather - gathered - gathering

Note: in two syllable words ending in one vowel followed by one "l", the "l" is doubled even if the last syllable is unstressed:

Travel - travelled - travelling

Fourthly, only some letters are doubled.

			B - rob - robbed - robbing
			D - bid - bidded - bidding
			G - dig - digging
			L - travel - travelled - travelling
			M - swim - swimming
			N - plan - planned - planning
			P - shop - shopped - shopping
			R - deter - deterred - deterring
			T - bet - betted - betting

For more information on doubling consonants, see:

http://english-zone.com/spelling/doubles.html

http://www.spelling.hemscott.net/suffix4A.html

http://www.gardenofpraise.com/spell1.htm

For quizzes on doubling consonants, see:

http://www.spelling.hemscott.net/suffix4B.html

http://webster.commnet.edu/cgi-shl/quiz20.pl/spelling_quiz3.htm

http://www.aitech.ac.jp/~iteslj/quizzes/9807/km-doubles.html