Editing Quadrilateral (section)

==Skew quadrilaterals==
{{See also|Skew polygon}}
[[File:Disphenoid tetrahedron.png|260px|thumb|The (red) side edges of [[tetragonal disphenoid]] represent a regular zig-zag skew quadrilateral]]

A non-planar quadrilateral is called a '''skew quadrilateral'''. Formulas to compute its dihedral angles from the edge lengths and the angle between two adjacent edges were derived for work on the properties of molecules such as [[cyclobutane]] that contain a "puckered" ring of four atoms.<ref>{{cite journal |first1=M. P. |last1=Barnett |first2=J. F. |last2=Capitani |title=Modular chemical geometry and symbolic calculation |journal=International Journal of Quantum Chemistry |volume=106 |issue=1 |pages=215–227 |year=2006 |doi=10.1002/qua.20807 |bibcode=2006IJQC..106..215B }}</ref> Historically the term '''gauche quadrilateral''' was also used to mean a skew quadrilateral.<ref>{{cite journal |last=Hamilton |first=William Rowan |url=http://www.maths.tcd.ie/pub/HistMath/People/Hamilton/Gauche/Gauche1.pdf |title=On Some Results Obtained by the Quaternion Analysis Respecting the Inscription of "Gauche" Polygons in Surfaces of the Second Order |journal=Proceedings of the Royal Irish Academy |volume=4 |year=1850 |pages=380–387 }}</ref> A skew  quadrilateral together with its diagonals form a (possibly non-regular) [[tetrahedron]], and conversely every skew quadrilateral comes from a tetrahedron where a pair of opposite [[edge (geometry)|edge]]s is removed.