Large circular displays rotating around the line of sight produce an illusion in which the seen orientation of the true vertical is shifted in a direction opposite to the display’s motion. Two experiments were performed to determine whether the magnitude of this illusory tilt is a function of the area of display elements, of their boundary length, or of their spatial frequency. In Experiment 1, 12 subjects viewed each of nine displays across which the number and area of the circular elements were independently varied. Three of the displays were equated for the area of their elements. The results suggested that tilt magnitude and onset latency could be explained by a boundary length effect. A second experiment tested eight subjects on two displays, equated for element boundary length but differing in the spatial frequency of the elements. The displays produced closely similar illusory trite corroborating the view that, within broad limits, element boundary length—and not spatial frequency or area—determines the size and onset latency of illusory tilt. A third experiment confirmed previous research in finding greater tilt and more rapid onset with more peripherally projected displays.