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These zig-zags are perfectly formed on a precisely square image of exact inch dimensions but have 45 degree diagonal lines crossing each of the boundary edges (contrasted against a matching one in each case which doesn't) to show the squashing damage done by the application of Spoonflower's rulers.
Using the link given in the embed text under the main image, you can see Spoonflower's own repeated image view before squashing and check that the damage is not done by their original image reduction of my perfect repeat.