This project showed that speakers use duration to highlight and demarcate meaningful units. In English, this is accomplished by lengthening prominent syllables, and through initial and final lengthening, which can occur at word, phrase, and utterance boundaries. Additional findings showed that under some circumstances, final lengthening can occur at morpheme boundaries, within words. An analysis of lengthening patterns found in this project and in the literature is consistent with the view that speakers use prosodic structure and associated durational correlates to highlight and demarcate words that are unpredictable from linguistic and pragmatic context. On this view, speakers use prosodic structure to make speech easier for listeners to understand. When planning speech, speakers balance the goal of making speech understandable with other potential goals, such as speaking quickly, or in rhythm.