This study investigated usability of graphical icons in the design of human-computer interfaces. A simple additive model of icon acquisition time, including terms for Index of Difficulty, mode of icon array, and log2 (number of icons) explained 95% of the variance in mean times covering all 36 conditions over 4 laboratory experiments conducted in the study. The acquisition of icons in graphical menus was subject to Fitts' Law. Where the dialog box only appeared on the screen when an action was initiated, acquisition time included a choice time that was additive with movement time. When a choice was required during the task in addition to a movement, both the number of icons and their configuration affected the choice time. Design implications are that the number of icons should be minimized in pop-up menus and that the icons should be arranged in a manner reflective of the shape of the useful field of view. Frequently used menus or icon arrays should be permanently visible to minimize performance time (e....