Three-dimensional visual scenes project onto the retina of the eye as two-dimensional images. The third dimension, depth, is projected as subtle differences between left and right retinal images. As early as the 1830s, stereoscopic depth perception was shown to depend on horizontal disparities between these images. To detect disparity, the visual system must match corresponding parts of the two retinal images. To identify the stimulus elements used in stereo matching, I applied a disparity-adaptation technique to visual patterns whose one-dimensional components and two-dimensional features have very different disparities. Surprisingly, the adaptors that are effective in altering depth perception appear widely separated in depth from the patterns they adapt. I conclude that stereo matching occurs in all directions of two-dimensional space and that one-dimensional components are the stimulus primitives, the fundamental elements of stereo matching. This is a reversal of the classical view of stereo correspondence as a one-dimensional (horizontal) matching of monocular two-dimensional features.