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  "body": "\n## Summary\n\nA Saccheri quadrilateral is a quadrilateral with two equal sides perpendicular to a common base.\nGiovanni Gerolamo Saccheri studied it in 1733 in an attempt to prove the [[parallel postulate|2PC3XY0PYZ]] from the other axioms.\nIts two top angles are equal, and they are right in [[Euclidean geometry|KRZD96GSVC]] and acute in [[hyperbolic geometry|GN4N94GQ0Z]].\nSo it measures how far an [[absolute plane|VKFPCSRNDH]] is from being Euclidean.\n\n## Explanations\n\n### L1 human\n\nDraw a segment as a base, raise two posts of equal length at right angles at its ends, and join the tops of the posts.\nOn a flat sheet you get a rectangle, so the top angles are right angles.\nSaccheri asked whether the top angles could be smaller than right angles without a contradiction.\nThey can: this happens in hyperbolic geometry, although Saccheri himself believed he had ruled it out.\n",
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