Quadrilaterals
- Aubel's Theorem
- Bicentric Quadrilateral
- Bimedian
- Brahmagupta's Formula
- Brahmagupta's Theorem
- Brahmagupta's Trapezium
- Bretschneider's Formula
- Complete Quadrangle
- Complete Quadrilateral
- Cyclic Quadrangle
- Cyclic Quadrilateral
- Diagonal Points
- Diagonal Triangle
- Diamond
- Equilic Quadrilateral
- Fano's Axiom
- Fuss's Problem
- Gauss-Bodenmiller Theorem
- Golden Rhombus
- Hesse's Theorem
- Isosceles Trapezium
- Isosceles Trapezoid
- Kite
- Léon Anne's Theorem
- Lozenge
- Medial Parallelogram
- Nine-Point Conic
- Orthocentric Centroid
- Orthocentric Quadrangle
- Orthocentric Quadrilateral
- Orthopolar Line
- Parallelogram
- Poncelet's Coaxal Theorem
- Ptolemy Inequality
- Ptolemy's Theorem
- Quadrangle
- Quadrilateral
- Quadrilateral Tiling
- Rational Quadrilateral
- Rectangle
- Rhomb
- Rhomboid
- Rhombus
- Right Trapezoid
- Skew Quadrilateral
- Square
- Strombus
- Sylvester's Four-Point Problem
- Tangential Quadrilateral
- Trapezium
- Trapezoid
- van Aubel's Theorem
- Varignon Parallelogram
- Varignon's Theorem
- Watt's Parallelogram
- Wittenbauer's Parallelogram
- Wittenbauer's Theorem