The key grouping structures relevant to metrical stress theory are the categories of the prosodic hierarchy. Prosodic categories can be divided into two types: interface categories and rhythmic categories. The interface categories are the utterance, the intonational phrase, the phonological phrase, and the prosodic word. The rhythmic categories are the foot, the syllable, and the mora. The key principles governing prosodic grouping are Constituency, Strict Succession, and Headedness. Constituency insists that prosodic groupings occur in the dominance relationship specified by the prosodic hierarchy. Strict Succession insists that phonological representations not skip prosodic categories moving lower to higher in hierarchy. Headedness insists that every instance of a prosodic category designate one of its immediate constituents as its head (its most prominent constituent). The combination of Headedness and Strict Succession insists that phonological representations not skip prosodic categories moving in the either direction, either lower to higher or higher to lower. Two special configurations play key roles in the theory: recursion and overlap. Interface categories may exhibit recursion, but recursion of rhythmic categories is prohibited by the Simple Layering condition. Instances of the same prosodic category may overlap so that they share a constituent.