The Parallelogram
Two red diagonals in two parallelograms, a big one and a small one.
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Two red diagonals in two parallelograms, a big one and a small one.
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A line takes the size of its shape
The two diagonals are exactly the same length. Each is seen as part of the shape around it, and the size of the whole leaks into the judgement of the part: the diagonal of the big parallelogram borrows its bigness, the one in the small parallelogram its smallness.
Friedrich Sander described it in 1926. Take the parallelograms away and what is left is a plain V with two equal arms.