*-iz (adjective *-izô)
- -er. Forms the comparative of certain adverbs.
This was the normal comparative suffix for all i-, u- and ja-stem adverbs. The regular a-stem adverbs sometimes used this suffix, and sometimes the related suffix *-ōz without any predictable rule.
Most of these descendants eventually replaced the suffix with the equivalent from the adjectives, because the older adverbial suffix was no longer distinctive after the loss of final *-iz.