Undergraduate students often have a choice about the modality in which they complete a particular course (in-person or online), and their decisions can be more complex than scholarship suggests. Most studies of modality choice have relied exclusively on closed-form, quantitative surveys; and this approach has denied students the opportunity to correct or complicate researchers' assumptions about their decision-making. We present findings from one of the largest and most comprehensive studies yet conducted on undergraduates' choice of course modality, which provided students with the opportunity to explain their choices in their own words. Our data reveal complexities of undergraduates' modality choice not mentioned previously in the literature, and open new avenues for research. They also have implications for course design during the period of COVID-19, by identifying features of in-person courses that instructors may wish to replicate in their emergency remote teaching for in-person students suddenly forced online.