Many modifications attach at the front of a word. A prefix is an affix placed before a root or stem to alter its meaning or create a new word.

English prefixes such as un-, re-, and pre- are highly productive. Prefixes may encode negation, repetition, degree, or spatial relations, among other notions. They generally do not change the word class of the base as often as some suffixes do, though exceptions exist.

Recognizing prefixes helps readers decode unfamiliar vocabulary. Historical processes can fuse prefixes so tightly that speakers no longer segment them. Cross-linguistic surveys show prefixes as one of the major positional types of affixation worldwide.

• Affix attached before a root or stem
• Alters meaning or derives new words
• Commonly marks negation, degree, or relation
• Supports vocabulary decoding for learners