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  "body": "\n## Definition\n\nLet $A$, $B$, $C$, $D$ be four points of an [[absolute plane|VKFPCSRNDH]].\nAssume that $C$ and $D$ lie on the same side of the line $AB$.\nThe quadrilateral $ABCD$ is a *Saccheri quadrilateral* with base $AB$ if:\n\n1. the [[angles|0GJAJC6TNZ]] $\\angle DAB$ and $\\angle ABC$ are right angles;\n2. the legs $AD$ and $BC$ are congruent.\n\nThe side $CD$ is the summit, and the angles $\\angle BCD$ and $\\angle CDA$ are the summit angles.\n\n## Remarks\n\nExample: in $\\R^2$, the points $A = (0,0)$, $B = (2,0)$, $C = (2,1)$, $D = (0,1)$ form a Saccheri quadrilateral, since both base angles are right and both legs have length $1$.\nNon-example: $A = (0,0)$, $B = (2,0)$, $C = (2,1)$, $D = (0,3)$ has right base angles but legs of lengths $1$ and $3$, so clause 2 fails.\nIn every absolute plane the summit angles are congruent, and the segment joining the midpoints of base and summit is perpendicular to both.\nThe summit angles are right in the Euclidean plane and acute in the hyperbolic plane.\n",
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