RULE I.—FINAL F, L, OR S.
Monosyllables ending in f, l, or s, preceded by a single vowel, double the final consonant; as staff, mill, pass—muff, knell, gloss—off, hiss, puss.
EXCEPTIONS.—The words clef, if, and of, are written with single f; and as, gas, has, was, yes, his, is, this, us, pus, and thus, with single s. So bul, for the flounder; nul, for no, in law; sol, for sou or sun; and sal, for salt, in chemistry, have but the single l.
OBS.—Because sal, salis, in Latin, doubles not the l, the chemists write salify, salifiable, salification, saliferous, saline, salinous, saliniform, salifying, &c., with single l, contrary to Rule 3d. But in gas they ought to double the s; for this is a word of their own inventing. Neither have they any plea for allowing it to form gases and gaseous with the s still single; for so they make it violate two general rules at once. If the singular cannot now be written gass, the plural should nevertheless be gasses, and the adjective should be gasseous, according to Rule 3d.
RULE II.—OTHER FINALS.
Words ending in any other consonant than f, l, or s, do not double the final letter; as, mob, nod, dog, sum, sun, cup, cur, cut, fix, whiz.
EXCEPTIONS.—We double the consonant in abb, ebb, add, odd, egg, jagg, ragg, inn, err, burr, purr, butt, buzz, fuzz, yarr, and some proper names. But we have also ab (from) and ad (to) for prefixes; and jag, rag, in, bur, and but, are other words that conform to the rule.
RULE III.—DOUBLING.
Monosyllables, and words accented on the last syllable, when they end with a single consonant preceded by a single vowel, or by a vowel after qu, double their final consonant before an additional syllable that begins with a vowel: as, rob, robbed, robber; fop, foppish, foppery; squat, squatter, squatting; thin, thinner, thinnest; swim, swimmer, swimming; commit, committeth, committing, committed, committer, committees; acquit, acquittal, acquittance, acquitted, acquitting, acquitteth.
EXCEPTIONS.—1. X final, being equivalent to ks, is never doubled: thus, from mix, we have mixed, mixing, and mixer. 2. When the derivative retains not the accent of the root, the final consonant is not always doubled: as, prefer', pref'erence, pref'erable; refer', ref'erence, ref'erable, or refer'rible; infer', in'ference, in'ferable, or infer'rible; transfer', a trans'fer, trans'ferable, or transfer'rible. 3. But letters doubled in Latin, are usually doubled in English, without regard to accent, or to any other principle: as, Britain, Britan'nic, Britannia; appeal, appel'lant; argil, argil'laus, argilla'ceous; cavil, cav'illous, cavilla'tion; excel', ex'cellent, ex'cellence; inflame', inflam'mable, inflamma'tion. See Observations 13 and 14, p. 199.