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Monosyllables and other 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 letter before a suffix beginning with a vowel: as, rob + ed = robbed; fop + ish = foppish; squat + er = squatter; prefer' + ing = prefer'ring.

Exceptions.—X final, being equivalent to ks, is never doubled; and when the derivative does not retain the accent of the root, the final consonant is not always doubled: as, prefer' + ence = pref'erence.