Horizontal lines run left to right, parallel to the horizon, with slope 0 and equation y = k. Vertical lines run top to bottom, perpendicular to the horizon, with undefined slope and equation x = k.

Morton A. Heller, Amy Smith, Rita Schnarr, Jennifer Larson and Soledad Ballesteros The present study compared the strength of the horizontal–vertical illusion in blindfolded sighted people with raised ...