When disparate images are shown to the two eyes, the perceived outcome may be fusion (single vision), suppression of one eye's view, or diplopia (double vision). We have re-examined the wayin which these perceptions depend on binocular disparity and spatial scale (eg Schor et al, 1984Vision Research24661 ^ 665) in an attempt to understand better the underlying spatial mechanisms and their interocular interactions. We used a novel 3-choice method to distinguish fusion,suppression and diplopia more clearly. Single, Gaussian-blurred, horizontal edges were presentedto each eye (30% contrast, 0.2 s) at various vertical disparities and with blurs B - 1 - 32 min of arc.Observers indicated '1 central edge', '1 offset edge' or '2 edges'. As disparity increased (from 0 to 8B),the proportion of fusion responses ('1 central edge') fell monotonically, and the fusional disparity range was nearly proportional to edge blur. The other responses did not scale with blur: at large disparities (44B), suppression was much more likely for small blurs, while diplopia dominated at large blurs. Data were fitted with a descriptive model in which the disparity range for fusionwas2:5B, while the disparity range for suppression was more than 10B at small blurs, falling to about 2.5B at large blurs. Thus, the range of fusion increases in proportion to the spatial scale of image features, but the range of suppression may be closer to being a constant disparity. Diplopia occurs - especially at large disparities and large blurs - when both fusion and suppression fail.