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Square  

A regular quadrilateral (tetragon)  
Type  Regular polygon 
Edges and vertices  4 
Schläfli symbol  {4} 
Coxeter diagram  
Symmetry group  D_{4}, order 2×4 
Internal angle (degrees)  90° 
Dual polygon  self 
Properties  convex, cyclic, equilateral, isogonal, isotoxal 
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Square  

A regular quadrilateral (tetragon)  
Type  Regular polygon 
Edges and vertices  4 
Schläfli symbol  {4} 
Coxeter diagram  
Symmetry group  D_{4}, order 2×4 
Internal angle (degrees)  90° 
Dual polygon  self 
Properties  convex, cyclic, equilateral, isogonal, isotoxal 
In geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90degree angles, or right angles).^{[1]} It can also be defined as a rectangle in which two adjacent sides have equal length. A square with vertices ABCD would be denoted ABCD.
The square is the n=2 case of the families of nhypercubes and northoplexes.
A convex quadrilateral is a square if and only if it is any one of the following:^{[3]}^{[4]}
The perimeter of a square whose four sides have length is
and the area A is
In classical times, the second power was described in terms of the area of a square, as in the above formula. This led to the use of the term square to mean raising to the second power.
The area can also be calculated using the diagonal d according to
In terms of the circumradius R, the area of a square is
and in terms of the inradius r, its area is
A convex quadrilateral with successive sides a, b, c, d is a square if and only if ^{[5]}^{:Corollary 15}
The coordinates for the vertices of a square centered at the origin and with side length 2 are (±1, ±1), while the interior of the same consists of all points (x_{i}, y_{i}) with −1 < x_{i} < 1 and −1 < y_{i} < 1.
The equation
describes the boundary of a square of side 2, centered at the origin. This equation means "x^{2} or y^{2}, whichever is larger, equals 1." The circumradius of this square (the radius of a circle drawn through the square's vertices) is half the square's diagonal, and equals . Then the circumcircle has the equation
Alternatively the equation
can also be used to describe the boundary of a square with center coordinates (a, b) and a horizontal or vertical radius of r.
The animation at the right shows how to construct a square using a compass and straightedge.
A square is a special case of a rhombus (equal sides, opposite equal angles), a kite (two pairs of adjacent equal sides), a parallelogram (opposite sides parallel), a quadrilateral or tetragon (foursided polygon), and a rectangle (opposite sides equal, rightangles) and therefore has all the properties of all these shapes, namely:^{[6]}
Every acute triangle has three inscribed squares (squares in its interior such that all four of a square's vertices lie on a side of the triangle, so two of them lie on the same side and hence one side of the square coincides with part of a side of the triangle). In a right triangle two of the squares coincide and have a vertex at the triangle's right angle, so a right triangle has only two distinct inscribed squares. An obtuse triangle has only one inscribed square, with a side coinciding with part of the triangle's longest side.
The fraction of the triangle's area that is filled by the square is no more than 1/2.
Squaring the circle is the problem, proposed by ancient geometers, of constructing a square with the same area as a given circle by using only a finite number of steps with compass and straightedge.
In 1882, the task was proven to be impossible, as a consequence of the Lindemann–Weierstrass theorem which proves that pi (π) is a transcendental number, rather than an algebraic irrational number; that is, it is not the root of any polynomial with rational coefficients.
In nonEuclidean geometry, squares are more generally polygons with 4 equal sides and equal angles.
In spherical geometry, a square is a polygon whose edges are great circle arcs of equal distance, which meet at equal angles. Unlike the square of plane geometry, the angles of such a square are larger than a right angle. Larger spherical squares have larger angles.
In hyperbolic geometry, squares with right angles do not exist. Rather, squares in hyperbolic geometry have angles of less than right angles. Larger hyperbolic squares have smaller angles.
Examples:
Six squares can tile the sphere with 3 squares around each vertex and 120degree internal angles. This is called a spherical cube. The Schläfli symbol is {4,3}.  Squares can tile the Euclidean plane with 4 around each vertex, with each square having an internal angle of 90°. The Schläfli symbol is {4,4}.  Squares can tile the hyperbolic plane with 5 around each vertex, with each square having 72degree internal angles. The Schläfli symbol is {4,5}. In fact, for any n ≥ 5 there is a hyperbolic tiling with n squares about each vertex. 
The K_{4} complete graph is often drawn as a square with all 6 edges connected. This graph also represents an orthographic projection of the 4 vertices and 6 edges of the regular 3simplex (tetrahedron).
Wikimedia Commons has media related to Squares (geometry). 
