Previous work suggested that elongation, horizontal area, and the boundary material might influence impressions of enclosure. This article describes an experiment in which these three factors were crossed. Ratings of open/enclosed by 15 respondents on 16 plazas indicated that the elongation ratio (range: 1:1 up to 1:1.8) had the strongest effect on impressions of enclosure (18% of variance). Horizontal area (12.25 m2 vs 49 m2) area accounted for 3% of variance. The amount of variance attributable to boundary material (plants vs walls) was an order of magnitude smaller (0.3%).