Extrametricality

A phonological unit (for instance a segment, mora, syllable, foot, affix or word) is called extrametrical if it is ignored by a (commonly: metrical) phonological rule; that is, it is treated as if it were not there. Hayes (1982) argues that extrametricality can be assigned by rule and is subject to the Peripherality Condition : extrametrical elements are always peripheral (found at one edge of the domain under consideration) and they lose their marking if they are not at the edge of the stress domain in the next phonological cycle.

Example
in English nouns the final syllable does not count for stress assignment and can be considered as extrametrical (indicated by angled brackets) (cf. Améri&lt;ca&gt; with antepenultimate stress, not *Ameríca). In English derived adjectives the adjectival suffix is extrametrical. In paréntal main stress is penultimate; the extrametricality of the final syllable of the noun párent is lost in the derived word by the Peripherality Condition and the stress rules reapply. Schematized in the grid:

a.           b.                      * *  .          *  .    .   --&gt;  * *. pa&lt;rent&gt;      pa&lt;rent&gt;&lt;al&gt;     paren&lt;tal&gt;

Link
Utrecht Lexicon of Linguistics