The finest human stereoacuity, which in some gifted individuals amounts to the detection of binocular disparities of less than 5 arc sec, is found with isolated vertical target lines 10-15 min of arc in length. Summation along the vertical dimension of the lines is physiological in origin, and is not due to probability summation of disparity signals from multiple point targets. What is being summed is not the quantity of light, but rather information about its distribution--positional signals leading to finer judgments of disparity. Increasing target length beyond 20 arc min produces little improvement in disparity thresholds because stereoacuity decreases at even small eccentricities. The threshold at 30 min away from the fixation point is nearly twice its value at the very center of the fovea. Fine stereoacuity has one additional constraint: the compared features should be disjoint. Connecting lines between test and reference targets can increase the stereo threshold markedly.