Although the shift from page to screen has dramatically redefined conceptions of writing, very little is known about how youth compose with multiple modes in digital environments. Integrating multimodality and multiliteracies theoretical frameworks, this comparative case study examined how urban twelfth-grade students collaboratively composed across three multimodal projects when responding to and analyzing literature. Data sources included screen capture and video observations, student design interviews, written reflections, and multimodal products. Findings revealed that multimodal composing was a complex, dynamic, and varied process mediated by the interaction of multiple factors. Students exhibited modal preferences when working with open and flexible digital tools--spending a majority of time working with that particular mode and relying on it to carry the communicative weight of their compositions. The development of multimodal composing timescapes for this study provided new insights into students' rapid and frequent cross-modal traversals as they worked on their digital projects.