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  "body": "\n## Summary\n\nA Lambert quadrilateral is a quadrilateral with three right angles.\nJohann Heinrich Lambert studied it in the 1760s in an attempt to prove the [[parallel postulate|2PC3XY0PYZ]].\nIts fourth angle is right in [[Euclidean geometry|KRZD96GSVC]] and acute in [[hyperbolic geometry|GN4N94GQ0Z]].\nHalf of a [[Saccheri quadrilateral|5SJVH0BHJE]], cut along the line joining the midpoints of base and summit, is a Lambert quadrilateral.\n\n## Explanations\n\n### L1 human\n\nDraw a shape with four sides in which three corners are square corners.\nOn a flat sheet the fourth corner is forced to be square too, so you get a rectangle.\nIn hyperbolic geometry the fourth corner is sharper than a right angle.\nLambert worked out many consequences of a sharp fourth corner and noticed that they look like geometry on a sphere of imaginary radius.\n",
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