The word consonant refers to the phonetic sound produced by occluding with or without releasing (p, b; t, d; k, g), diverting (m, n, ng), or obstructing (f, v; s, z, etc.) the flow of air from the lungs. From grade school, you remember the vowels as a, e, i, o, u, and sometimes y. All other letters are consonants. When spelling, sometimes you'll double a consonant ending a word before adding a suffix, as I just did when I doubled the consonant “l” in spelling. Here are some handy rules from The Columbia Guide to Standard American English:
1. Words that end in a single vowel plus a single consonant usually double the final consonant before adding a suffix that begins with a vowel: stop becomes stopped, stopping, stopper, and unstoppable. Thus snip becomes snipper, but snipe becomes sniper.
2. Most words that end in two consonants do not ordinarily double the final consonant before a suffix: print becomes printed, printing, and printer.
3. If the suffix begins with a consonant instead of a vowel, the final consonant of the base word stays single: ship becomes shipment and clap becomes claptrap.
4. Words of two and more syllables that are stressed on the final syllable normally double the final consonant before adding a suffix: infer becomes inferred and inferring. But two-syllable words stressed on the final syllable do not double the final consonant when the suffix begins with a consonant: regret becomes regretting but regretful. And words stressed on the final syllable but ending with two consonants or with a vowel do not double the consonant: predict becomes predicting and predicted.
5. Words that end in ‑c usually add ‑k before the suffix: panic becomes panicking; picnic, picnicked.
6. In words of more than one syllable ending in a consonant, especially ‑l, the English generally double the final consonant, and Americans generally do not.
Here are some examples of Rule 6: