Vanishing Line - where did it go? Math magic illusion puzzle

Look at the 2 pieces of wood and you will clearly see 13 horizontal lines. Yet flip the frame upside down and there are only 12 lines! How can this be? My son calls this the "Sobriety Test", yet there is an interesting math basis for the puzzle which is explained in the video. It is a wonderful illusion that I got the idea to make from one of Martin Gardner

Which shape has more of its' area shaded? : r/puzzles

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A selection of vanishing puzzles

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Spec Ops: The Line' Disappeared. It Won't Be the Last Beloved Game