Lambert quadrilateral

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A Lambert quadrilateral is a four sided figure. It can only exist in non-Euclidean geometry. It has a base, AB, two unequal legs standing at right angles to it, AC and BD, and the summit, CD, meets one of the two legs at a right angle and the other leg at an acute angle. It is composed entirely of straight lines.

The Lambert quadrilateral was realized while Johann Lambert was studying Saccheri quadrilaterals.