Not every affix sits neatly at an edge. An infix is an affix inserted inside a base word rather than attached before or after it.

English has few true infixes, mostly playful insertions in emotive speech, but many languages use infixes for tense, focus, or derivation. Tagalog, for example, is well known for productive infixation. Identifying the insertion point requires morphological analysis of the base.

Infixes challenge simple models of word building that assume only prefixes and suffixes. Field grammars document infix position and function carefully. Historical processes can turn once-separate elements into infixes through reanalysis of boundaries.

• Affix inserted within a word
• Productive in some languages, marginal in others
• Marks grammatical or derivational categories
• Requires analysis of internal word structure