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

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

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

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