RULE VI.
Monosyllables and words having the primary accent on the last syllable, when they end with a single consonant preceded by a single vowel, double their final consonant before an additional syllable that begins with a vowel; as, wet, wetter, wettest, wetting, wetted; drum, drumming, drummed; dispel, dispelling, dispelled.
Exception.—A final x, or the s in gas, should not be doubled; as, fix, fixes, fixed, fixing; annex, annexing; gases, gasefy.
Remark I.—U after q is never reckoned a part of a diphthong or triphthong; so that from quit are formed quitting, quitted; and from quag, quaggy.
Remark II.—This rule applies only to derivatives which retain the accent of their primitives, and not to such as in´ferable, in´ference, pref´erable, pref´erence, ref´erable, and ref´erence, from infer, prefer, and refer. To the forms infer´rible, refer´rible, which are sometimes met with, the general rule applies. Transfer´able, from transfer, is an exception to the general rule; the regular form transfer´rible is not often used. Although parallel´ogram, from par´allel, and modal´ity from mo´dal, remove the primary accent to the point of duplication, they do not double the final l. See Remark II. under Rule VII.
RULE VII.
A final consonant is not doubled when it is preceded by a diphthong, when the primary accent is either not on, or not retained upon, the last syllable, or when the additional syllable begins with a consonant; as, beat, beating, beaten; dif´fer, dif´fering, dif´fered, dif´ference, dif´ferent; prefer´, pref´erence; refer´, ref´erence; fit, fitful, fitly, fitness; ben´efit, ben´efited, ben´efiting.
Exception I.—Compounds that remove the primary accent from the point of duplication retain the double letter; as, broad´-brimmed, heel´-tapping.
Remark I.—When ly is affixed to words ending in l, the l is not considered doubled; as in cool-ly, real-ly, gravel-ly, royal-ly.