The grammatical function of stress accent is categorized into morphological and demarcative subfunctions. We propose that these functions are realized by Optimality-Theoretic constraints on prosodic structure. The morphological function of stress is fulfilled by Identity Constraints, which require a derived form to preserve the prosodic structure of the base and the affix. Demarcative stress indicates a word boundary, and this function of stress is realized by Alignment Constraints. Stress shift is caused by transition which follows reranking of Alignment Constraints over Identity Constraints. We claim the ongoing stress shift demonstrates that the demarcative function predominates the morphological function of stress accent in Present-Day English.