A double comparative is a sentence that compares two things. Forming adjectives and adverbs in the comparative degree can be a little tricky.
Adjectives in the comparative degree end in -er, such as newer in the previous sentence “My car is newer than his car.”. Adverbs that end in -ly use “more” instead to form the comparative degree, as in more highly in the sentence “Mary speaks more highly of the iTouch than the iPod.”.
The most common error with double comparatives is the usage of both the -er ending and the word “more”. These sentences generally sound awkward when read out loud. Consider the following sentence:
“Mary speaks more highlier of the iTouch than the iPod.”
In this case it would be best to remove the -er and use the adverb in its comparative degree. Now, reconsider the sentence:
“Mary speaks more highly of the iTouch than the iPod.”