Visual-noise masks composed of randomly scattered letter fragments disrupt the recognition of briefly viewed letter arrays when the masks are presented immediately after these arrays. According to the feature-similarity assumption, maximal disruption should occur when the target letters and the letter fragments comprising the mask share a common stroke width. This assumption was disconfirmed in the present experiment. Two-, four-, and six-letter arrays, whose stroke width subtended a visual angle of 3-8', were viewed tachistoscopically in conjunction with visual-noise masks whose letter fragments subtended visual angles of 1-2', 1-9', 3-8', 6-6', or 9-7'. The most effective mask for all three letter-array lengths was the one composed of letter fragments subtending 6-6'. The results, together with those of previous experiments, are accounted for in terms of a neural line-detecting mechansim.