From the archives...
A customer wrote us to ask:
Could you tell me why we drop the final silent e when adding the suffix -ly to true?
What a great question! You are right that this word does not follow the suffixing rules; typically if the base word ends with a silent E and we are adding a consonant suffix, we keep the E and simply add a suffix after it (lovely, hateful, etc.). Truly and duly are true exceptions, among the 2% of English words with one or more sounds spelled in a way that breaks the rules. Words that break the suffixing rules are discussed on pages 74-75 of Uncovering the Logic of English.
Interestingly, the spelling is still fairly logical, though it adds its suffix in an unconventional way: The E is not needed in truly, since AEOU usually say their long sound at the end of a syllable (Rule 4). That is why there isn't an E in duty, tutor, etc. So the word truly is completely decodable in a way that follows the spelling rules even though the way the suffix was added is not.