Metrical phonology organizes syllables into feet (binary or ternary units) that carry stress. Trees represent relative prominence, with stressed syllables dominating unstressed ones. Feet are constructed left-to-right or right-to-left depending on the language, building successively larger prosodic domains. This account explains stress patterns, secondary stress, and the phonological effects of stress without needing language-specific stress rules.
Build metrical trees for words in several stress systems (English, Spanish, Finnish) and verify that the foot structure correctly predicts which syllables bear stress. Examine how foot type (iambic, trochaic) and directionality vary across languages.
From suprasegmental phonology, you know that stress is a property of syllables that involves a cluster of phonetic features — greater loudness, longer duration, and often higher pitch — and that stress patterns vary systematically across languages. You also know from syllable structure that syllables are organized into hierarchical constituents (onset, nucleus, coda) with the nucleus as the head. Metrical theory takes this hierarchical intuition and extends it upward: syllables group into feet, feet group into prosodic words, and prosodic words group into larger units. Stress, in this view, is not a rule-stipulated property of individual syllables but a consequence of where heads fall in this hierarchical structure.
The foundational unit is the metrical foot — a grouping of two (or sometimes three) syllables with one head (the strong or stressed syllable) and one or more dependents (weak or unstressed syllables). Two fundamental foot types are the trochee (strong-weak, SW) and the iamb (weak-strong, WS). English is predominantly trochaic: *PEA-nut*, *TA-ble*, *WIN-dow* all have the strong syllable first. Latin and Arabic stress systems are more iambic. The foot type is a property of the language's phonology, not of individual words, and it determines the default rhythm of the whole language.
Directionality of foot-building is the second key parameter. Languages construct feet either left-to-right or right-to-left from the edge of the word. English, for example, builds trochees from left to right: *ÁLA-bama* parses as (ÁLA)-(ba-MA) where the first foot is strong-weak and the second is weak-strong (actually English metrical structure is more complex, but the leftward bias is real). The combination of foot type and directionality predicts where primary stress falls in most words, and where secondary stress falls in longer words. Secondary stress arises because every foot has a head — even non-primary feet contribute a weaker beat. This is why *Ála-bàma* has both primary stress on the first syllable and a secondary stress on the third: the prosodic structure requires it, not an arbitrary rule.
The explanatory power of the metrical approach becomes clear when you compare it to the alternative: a list of stress rules. A purely rule-based account might say "stress the third-to-last syllable in Spanish" or "stress the antepenult if the penult is light." These rules work for regular cases but require additional stipulations for every exception, and they don't explain why the exceptions cluster the way they do. Metrical theory shows that most "exceptions" are not really exceptions — they reflect interaction between the regular foot structure and other phonological factors like syllable weight (the distinction between heavy syllables with long vowels or codas, and light syllables with short vowels). Heavy syllables attract stress because they are metrically stronger — they can function as the head of a foot more readily than light syllables. This weight-sensitivity is not a separate rule but a consequence of foot structure.
Metrical trees make the hierarchical relationships explicit. At the bottom, syllables are labeled strong (S) or weak (W) within feet. At the next level, feet are labeled strong or weak within the prosodic word. Primary stress falls on the strong syllable of the strong foot; secondary stress falls on the strong syllable of weak feet. The tree is a visual representation of the claim that stress is relational and hierarchical, not a fixed phonetic property. This has consequences beyond stress: the same metrical structure governs phenomena like syncope (vowel deletion in unstressed syllables), flapping in American English (where the /t/ in *butter* becomes a flap partly because it is in a weak syllable), and the prosodic environments for various phonological rules. Stress, in metrical theory, is not just about which syllables are louder — it is the organizing principle of the phonological word.