ABSTRACTSThere is some evidence that early medieval single‐light splayed windows were specifically designed for maximum optical efficiency. Some factors affecting the design of an individual window are examined from three points of view: (1) a brief theoretical consideration of the crude effects of size and angle of splay; (2) an abbreviated account of an analysis of the characteristics of actual windows surviving from the Anglo‐Saxon period; (3) an account of an experiment to determine by means of a scale apparatus the relative effects of window size, angle and type of splay, and surface finish of the fab