In linguistics, extrametricality is a tool for prosodic analysis of words in a language.
For example, in a language like classical Latin, where polysyllabic words never have stress on their final syllables, and the position of stress in a word is determined by looking at the penultimate and antepenultimate syllables only, it simplifies the linguistic formulation of the stress-assignment rules of Latin to say that the final syllable of a polysyllabic word is invisible to rules which determine stress.
Most typically, extrametricality affects one specified segment or prosodic unit (mora, syllable, etc.)
Final-segment or final-mora extrametricality can be invoked to account for the phenomenon that word-final syllables often count as "lighter" than other syllables of the same rime type for purposes of determining the position of stress in a word.
However, other analyses of these patterns which avoid the need for extrametricality are possible: it may be posited that Weight By Position on English applies only preconsonantally (although this leaves words ending in a long vowel and a coda cluster problematic); another analysis might propose that all English words end in a catalectic syllable, and the apparently word-final consonant(s) are parsed in the onset of this syllable.