Using recently developed image processing software, the presence of a wavy band of threads, occurring only in the weft direction of hand-loomed fabric, is vividly displayed in colour-coded maps of the local direction of the threads. Observation of a canvas-wide wavy group of threads labelled a ‘weft snake’ reliably establishes the warp–weft orientation of the painting’s canvas, which is a key ingredient in identifying canvases from the same original bolt. A few examples of weft snake detection are provided in a range of European paintings across countries and centuries. The occurrence of ‘weft snakes’ appears more often than residual selvedges for old master paintings, and is equally reliable in designating the fabric’s warp–weft orientation.