Example: unnecessary, unnerve, unnumbered

Example from my dictionary:

Words ending with silent e usually retain the e before a suffix beginning with a consonant. Example: word (arrange) + suffix (ment) = arrangement, there are, of course, exceptions to words.

Words ending in a single consonant preceded by a single vowel usually double the final consonant before a suffix beginning with a vowel. run + ing = running

If the word ends with 2 or more consontants, or if the final consonant is preceded by 2 vowels instead of 1, the rule does not apply. Example: debt + or = debtor

Source: The New Lexicon Webster’s Enclyclopedia Dictionary, Canadian Edition, 1988