(a) Demonstration of crowding: fixating on the red ‘−‘ it should be easy to identify the letter s on the left; the equidistant s on the right, which is flanked (crowded) by other letters, is much harder to identify. When fixation is shifted to the green ’+’, the formerly-crowded s becomes easier to identify. (b) The extent of crowding (“crowding zone”, orange polygons) can be estimated by measuring target-identification performance at peripheral locations (demarcated by *) with flankers placed at various relative positions around the target. The estimated zones have three robust signatures: they scale up linearly with eccentricity of the target (Bouma’s Law); they are markedly elongated along the axis connecting the target to the fovea (radial axis); when tested with a single flanker (green dotted contour), as opposed to a pair (orange solid contours), flankers that are more eccentric than the target are more effective in crowding the target than are flankers that are less eccentric. (a: adapted from REF [10]; b: adapted from REF [9])