Whole-class discussions, in which students share and refine ideas with others, can now take place in online forums. In face-to-face classrooms, previous research has identified conversational techniques, such as open-ended questions and uptake of what others have said, that can promote dialogic, whole-class discussions. However, few studies have examined how students' participation can be enabled and constrained by the visual design of online forums. This article evaluates three online conversations among students of different disciplines and geographic regions at secondary, postsecondary, and graduate levels, evaluating the success of each example based first on conversational techniques and then on the visual design of the online forum. Rhetorical principles such as contrast, repetition, alignment, and proximity may help teachers evaluate and redesign potential online forums for more effective online discussions, including how classroom setup, routines, and multimedia can shape students' participation.