In certain circumstances, English "doubles", the final consonant letter of a word or proper noun is "doubled" before a vowel-initial suffix in certain circumstances.
The most common suffixes that regularly causecircumstances for this kind of doubling are -ing, -ed, -er, -able, -est, -y; less:
verb inflection with -ing or -ed
adjective inflection with -er or -est
creating an agent noun ending in -er by derivation from a verb
creating an adjective ending in -able by derivation from a verb
creating an adjective ending in -y by derivation from some word (usually a noun)
Less frequent derivational suffixes that can cause doubling are -ist and -ish, as in druggist and priggish.
Note that double consonants occur in other contexts, where they may exist for other reasons. For example, double consonants may arise simply from the concatenation of two words in a compound, such as lamppost. Double consonants also occur in words that were derived before they entered English, such as cancellation from Latin cancellatio(nem) or inflammation from Latin inflammatio(nem). Those other kinds of "double consonants" don'tdo not follow the same rules given in this post.
OtherAll other letters are typically subject to the doubling rule. The, but the rule is less clearly established for letters that are rare or extremely rare in word-final position (v j q), but we do see doubled spellings in e.g. shivved, so v at least seems to be subject to the rule.
- With v, shiv and rev form shivved and revved.
A letter must be word-final in the base word to be doubled: there are no words that remove a final "silent e" and also double the preceding consonant letter. So we never double the consonant in words like giving, having, promising, disciplining.*
Doubling is only possible when the consonant letter comes directly after a single vowel letter that by itself represents the vowel of the last syllable. This means that:
Doubling does not occur after a vowel digraphs like ea ordigraph ai(a pair of vowel letters that corresponds to a single vowel sound**), regardless of how they arewhether it is pronounced as a "long vowel" or as a "short vowel".** Words like headed, sweated, plaited, flooded are spelled with single consonants.
Doubling may occur after a single vowel letter preceded by qu (quipped) or by another vowel in hiatus (duetted).**
Doubling does not occur in words that end in more than one consonant letter, regardless of whether that isthey form a digraph (slashed, attached, graphed, pathed) or consonant cluster (twisted, acted).
Focussed/focussing is another exception to the stress-based rule: writing it with a double consonant is optional. I don't think this really fits into any general pattern of exceptions. Verbs ending in an unstressed syllable spelled with a single final s are uncommon, and none ofwhich might account for the others have -ssed spellings with anywhere nearvariability in the frequencyspelling of focussed. Bias can form biassed according to some dictionaries (e.g. AHD, Cambridge) but this seems to be extremely infrequent in modern texts.and a few other words:
The verb bus—which is monosyllabic, and therefore stressed—shows variation between the spellings bussed, bussing, with regular consonant doubling, and the exceptional spellings bused, busing.
Bias can form biassed according to some dictionaries (e.g. AHD, Cambridge) but this seems to be extremely infrequent in modern texts. The usual spelling is biased, without doubling.