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Adjacent unequal parallelograms containing two highlighted diagonals of equal length.
Look closely. Let the image settle.
Size & geometry / 1922 / 1926

Sander’s parallelogram

Equal diagonals look unequal inside different frames.

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Compare the two highlighted diagonals rather than the surrounding parallelograms. Their lengths match.

FOLLOW THE SOURCES

History, evidence & image credit

THE IMAGE IN THIS ARCHIVE

CountingPine

Original file & attribution · Public domain

Original image downloaded from Wikimedia Commons; the downloaded file is unchanged. Gallery previews may be resized or converted to WebP; animated previews show the first frame.

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Frequently asked questions

What is Sander’s parallelogram?

Equal diagonals look unequal inside different frames.

What should I look for in Sander’s parallelogram?

Compare the two highlighted diagonals rather than the surrounding parallelograms. Their lengths match.

What explains Sander’s parallelogram?

The surrounding angles and suggested spatial structure alter the lengths assigned to the diagonal segments. Perspective interpretation is one proposed contribution; geometric interactions provide another.

Another way to see it

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Adjacent unequal parallelograms containing two highlighted diagonals of equal length.

Sander’s parallelogram