Previously, generative phonologists and the American Structuralists represented prosodic prominence as a feature that applied to individual phonemes (segments) or syllables.
(1) Metrical trees allow us to change the stress pattern for a phrase by switching S and W sister nodes.
The same metrical structure would be used when the sentence has narrow focus on the word 'penicillin'; for example, if it was used in response to a question like "What do doctors use to treat that disease?".
However, we need a new metrical structure to put narrow focus on the word 'doctors', for example, if the phrase is used in response to the question "Who uses penicillin?"
The relations between prosodic constituents at different levels is commonly thought to be governed by the Strict Layer Hypothesis (SLH).
The various levels of the prosodic hierarchy are independently justified by the phonological phenomena that make reference to them.
[5] Similarly, in the Gorgia Toscana variety of Italian, the intonation phrase is the domain of a rule that changes voiceless plosives (/p/, /t/, /k/) between vowels into fricative consonants, like /θ/ (th) and /h/.
Indeed, a set of rules developed by Liberman and Prince[2] can be used to quite accurately predict stress in English words.
Their Lexical Category Prominence Rule states that the second node in a pair of sister nodes is labeled W unless one of a number of conditions are met, such as the node branching or dominating a particular suffix, in which case it is labeled S. Allowable tree structures and node labels for a particular word in Liberman and Prince's system are constrained by the two-value feature [± stress], which can be assigned to segments or syllables by separate rules that refer to the number and type of segments in the syllable and the syllable's position in the word.
The structure of the metrical grid explains a number of otherwise surprising features of prominence patterns in language.
Metrical grids were originally developed to handle a phenomenon that appears in some languages, including English, German, and Masoretic Hebrew, in which stress shifts to avoid a 'stress clash'.
Two syllables exhibit stress clash if there are two successive rows in the grid in which their columns are adjacent (i.e. there is no X between them).
Hayes (1981)[10] describes four metrical parameters which can be used to group languages according to their word-level stress patterns.
Empty terminal nodes correspond to rests or form part of a note that spans several beats.
Finally, metrical phonology is consistent with patterns of deaccenting in which accents can shift both left and right.