Bend gestures have a large number of degrees of freedom and therefore offer a rich interaction language. We propose a classification scheme for bend gestures, and explore how users perform these bend gestures along four classification criterion: location, direction, size, and angle. We collected 36 unique bend gestures performed three times by each participant. The results suggest a strong agreement among participants for preferences of location and direction. Size and angle were difficult for users to differentiate. Finally, users performed and perceived two distinct levels of magnitude. We propose recommendations for designing bend gestures with flexible displays.