In Euclidean plane geometry, a **quadrilateral** is a polygon with four edges (sides) and four vertices (corners). Other names for quadrilateral include **quadrangle** (in analogy to triangle), **tetragon** (in analogy to pentagon, 5-sided polygon, and hexagon, 6-sided polygon), and **4-gon** (in analogy to *k*-gons for arbitrary values of *k*). A quadrilateral with vertices , , and is sometimes denoted as .^{[1]}^{[2]}

The word "quadrilateral" is derived from the Latin words *quadri*, a variant of four, and *latus*, meaning "side".

Quadrilaterals are either simple (not self-intersecting), or complex (self-intersecting, or crossed). Simple quadrilaterals are either convex or concave.

The interior angles of a simple (and planar) quadrilateral *ABCD* add up to 360 degrees of arc, that is^{[2]}

This is a special case of the *n*-gon interior angle sum formula: (*n* − 2) × 180°.

All non-self-crossing quadrilaterals tile the plane, by repeated rotation around the midpoints of their edges.

The bimedians of a quadrilateral are the line segments connecting the midpoints of the opposite sides. The intersection of the bimedians is the centroid of the vertices of the quadrilateral.^{[13]}

The midpoints of the sides of any quadrilateral (convex, concave or crossed) are the vertices of a parallelogram called the Varignon parallelogram. It has the following properties:

- Each pair of opposite sides of the Varignon parallelogram are parallel to a diagonal in the original quadrilateral.
- A side of the Varignon parallelogram is half as long as the diagonal in the original quadrilateral it is parallel to.
- The area of the Varignon parallelogram equals half the area of the original quadrilateral. This is true in convex, concave and crossed quadrilaterals provided the area of the latter is defined to be the difference of the areas of the two triangles it is composed of.
^{[30]} - The perimeter of the Varignon parallelogram equals the sum of the diagonals of the original quadrilateral.
- The diagonals of the Varignon parallelogram are the bimedians of the original quadrilateral.

The two bimedians in a quadrilateral and the line segment joining the midpoints of the diagonals in that quadrilateral are concurrent and are all bisected by their point of intersection.^{[22]}^{:p.125}

In a convex quadrilateral with sides *a*, *b*, *c* and *d*, the length of the bimedian that connects the midpoints of the sides *a* and *c* is

*A*and*C*meet on diagonal*BD*, then the angle bisectors of*B*and*D*meet on diagonal*AC*.^{[29]}The bimedians of a quadrilateral are the line segments connecting the midpoints of the opposite sides. The intersection of the bimedians is the centroid of the vertices of the quadrilateral.

^{[13]}The midpoints of the sides of any quadrilateral (convex, concave or crossed) are the vertices of a parallelogram called the Varignon parallelogram. It has the following properties:

- Each pair of opposite sides of the Varignon parallelogram are parallel to a diagonal in the original quadrilateral.
- A side of the Varignon parallelogram is half as long as the diagonal in the original quadrilateral it is parallel to.
- The area of the Varignon parallelogram equals half the area of the original quadrilateral. This is true in convex, concave and crossed quadrilaterals provided the area of the latter is defined to be the difference of the areas of the two triangles it is composed of.
^{[30]} - The perimeter of the Varignon parallelogram equals the sum of the diagonals of the original quadrilateral.
- The diagonals of the Varignon parallelogram are the bimedians of the original quadrilateral.

The two bimedians in a quadrilateral and the line segment joining the midpoints of the d

The midpoints of the sides of any quadrilateral (convex, concave or crossed) are the vertices of a parallelogram called the Varignon parallelogram. It has the following properties:

The two bimedians in a quadrilateral and the line segment joining the midpoints of the diagonals in that quadrilateral are concurrent and are all bisected by their point of intersection.

^{[22]}^{:p.125}In a convex quadrilateral with sides

*a*,*b*,*c*and*d*, the length of the bimedian that connects the midpoints of the sides*a*and*c*isHe

Hence

^{[22]}^{:p.126}This is also a corollary to the parallelogram law applied in the Varignon parallelogram.

The lengths of the bimedians can also be expressed in terms of two opposite sides and the distance

*x*between the midpoints of the diagonals. This is possible when using Euler's quadrilateral theorem in the above formulas. Whence^{[21]}- corollary to the parallelogram law applied in the Varignon parallelogram.
The lengths of the bimedians can also be expressed in terms of two opposite sides and the distance

*x*between the midpoints of the diagonals. This is possible when using Euler's quadrilateral theorem in the above formulas. Whence^{[21]}