Saccheri quadrilateral
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A Saccheri quadrilateral is a four-sided figure. It can only exist in non-Euclidean geometry. It has a base, AB, two equal legs standing at right angles to it, AC and BD, and acute angles at the summit, CD. It is composed entirely of straight lines. It is named after Giovanni Gerolamo Saccheri.
A Saccheri quadrilateral is composed of two equal Lambert quadrilaterals.