In geometry, a square is a regular polygon quadrilateral, which means that it has four equal sides and four equal (90degree angles, or (100gradian angles or right angles). It can also be defined as a rectangle in which two adjacent sides have equal length. A square with vertices ABCD would be denoted .
Characterizations
A
Convex polygon quadrilateral is a square if and only if it is any one of the following:
[Zalman Usiskin and Jennifer Griffin, "The Classification of Quadrilaterals. A Study of Definition", Information Age Publishing, 2008, p. 59, .]

a rectangle with two adjacent equal sides

a rhombus with a right vertex angle

a rhombus with all angles equal

a parallelogram with one right vertex angle and two adjacent equal sides

a quadrilateral with four equal sides and four

a quadrilateral where the diagonals are equal and are the perpendicular bisectors of each other, i.e. a rhombus with equal diagonals

a convex quadrilateral with successive sides a, b, c, d whose area is $A=\; \backslash tfrac\{1\}\{2\}(a^2+c^2)=\backslash tfrac\{1\}\{2\}(b^2+d^2).$
[Josefsson, Martin, "Properties of equidiagonal quadrilaterals" Forum Geometricorum, 14 (2014), 129144.]
Properties
A square is a special case of a
rhombus (equal sides, opposite equal angles), a kite (two pairs of adjacent equal sides), a
trapezoid (one pair of opposite sides parallel), a
parallelogram (all 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:

The of a square bisection each other and meet at 90°

The diagonals of a square bisect its angles.

Opposite sides of a square are both parallel and equal in length.

All four angles of a square are equal. (Each is 360°/4 = 90°, so every angle of a square is a right angle.)

All four sides of a square are equal.

The diagonals of a square are equal.

The square is the n=2 case of the families of nhypercubes and n.

A square has Schläfli symbol {4}. A truncated square, t{4}, is an octagon, {8}. An alternated square, h{4}, is a digon, {2}.
Perimeter and area
The
perimeter of a square whose four sides have length
$\backslash ell$ is
 $P=4\backslash ell$
and the area A is
 $A=\backslash ell^2.$
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
 $A=\backslash frac\{d^2\}\{2\}.$
In terms of the circumradius R, the area of a square is
 $A=2R^2;$
since the area of the circle is $\backslash pi\; R^2,$ the square fills approximately 0.6366 of its circumscribed circle.
In terms of the inradius r, the area of the square is
 $A=4r^2.$
Because it is a regular polygon, a square is the quadrilateral of least perimeter enclosing a given area. Dually, a square is the quadrilateral containing the largest area within a given perimeter.[Chakerian, G.D. "A Distorted View of Geometry." Ch. 7 in Mathematical Plums (R. Honsberger, editor). Washington, DC: Mathematical Association of America, 1979: 147.] Indeed, if A and P are the area and perimeter enclosed by a quadrilateral, then the following isoperimetric inequality holds:
 $16A\backslash le\; P^2$
with equality if and only if the quadrilateral is a square.
Other facts

The diagonals of a square are $\backslash scriptstyle\; \backslash sqrt\{2\}$ (about 1.414) times the length of a side of the square. This value, known as the square root of 2 or Pythagoras' constant, was the first number proven to be irrational.

A square can also be defined as a parallelogram with equal diagonals that bisect the angles.

If a figure is both a rectangle (right angles) and a rhombus (equal edge lengths), then it is a square.

If a circle is circumscribed around a square, the area of the circle is $\backslash pi/2$ (about 1.5708) times the area of the square.

If a circle is inscribed in the square, the area of the circle is $\backslash pi/4$ (about 0.7854) times the area of the square.

A square has a larger area than any other quadrilateral with the same perimeter.

A square tiling is one of three regular tilings of the plane (the others are the equilateral triangle and the regular hexagon).

The square is in two families of polytopes in two dimensions: hypercube and the crosspolytope. The Schläfli symbol for the square is {4}.

The square is a highly symmetric object. There are four lines of reflectional symmetry and it has rotational symmetry of order 4 (through 90°, 180° and 270°). Its symmetry group is the dihedral group D_{4}.

If the inscribed circle of a square ABCD has tangency points E on AB, F on BC, G on CD, and H on DA, then for any point P on the inscribed circle,
 :$2(PH^2PE^2)\; =\; PD^2PB^2.$

If $d\_i$ is the distance from an arbitrary point in the plane to the ith vertex of a square and $R$ is the circumradius of the square, then
[Park, PooSung. "Regular polytope distances", Forum Geometricorum 16, 2016, 227232. http://forumgeom.fau.edu/FG2016volume16/FG201627.pdf]
 :$\backslash frac\{d\_1^4+d\_2^4+d\_3^4+d\_4^4\}\{4\}\; +\; 3R^4\; =\; \backslash left(\backslash frac\{d\_1^2+d\_2^2+d\_3^2+d\_4^2\}\{4\}\; +\; R^2\backslash right)^2.$
Coordinates and equations
The coordinates for the vertices of a square with vertical and horizontal sides, centered at the origin and with side length 2 are (±1, ±1), while the interior of this square consists of all points (
x_{i},
y_{i}) with and . The equation
 $\backslash max(x^2,\; y^2)\; =\; 1$
specifies the boundary of this square. 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 $\backslash scriptstyle\; \backslash sqrt\{2\}$. Then the circumcircle has the equation
 $x^2\; +\; y^2\; =\; 2.$
Alternatively the equation
 $\backslash leftx\; \; a\backslash right\; +\; \backslash lefty\; \; b\backslash right\; =\; r.$
can also be used to describe the boundary of a square with center coordinates ( a, b) and a horizontal or vertical radius of r.
Construction
The following animations show how to construct a square using a compass and straightedge. This is possible as 4 = 2
^{2}, a power of two.
Symmetry
The
square has Dih
_{4} symmetry, order 8. There are 2 dihedral subgroups: Dih
_{2}, Dih
_{1}, and 3
cyclic group subgroups: Z
_{4}, Z
_{2}, and Z
_{1}.
A square is a special case of many lower symmetry quadrilaterals:

a rectangle with two adjacent equal sides

a quadrilateral with four equal sides and four

a parallelogram with one right angle and two adjacent equal sides

a rhombus with a right angle

a rhombus with all angles equal

a rhombus with equal diagonals
These 6 symmetries express 8 distinct symmetries on a square. John Conway labels these by a letter and group order.[John H. Conway, Heidi Burgiel, Chaim GoodmanStrauss, (2008) The Symmetries of Things, (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275278)]
Each subgroup symmetry allows one or more degrees of freedom for irregular . r8 is full symmetry of the square, and a1 is no symmetry. d4, is the symmetry of a rectangle and p4, is the symmetry of a rhombus. These two forms are dual polygon of each other and have half the symmetry order of the square. d2 is the symmetry of an isosceles trapezoid, and p2 is the symmetry of a kite. g2 defines the geometry of a parallelogram.
Only the g4 subgroup has no degrees of freedom but can seen as a square with .
Squares inscribed in triangles
Every
acute triangle has three
inscribed figure 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
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 () is a transcendental number, rather than an algebraic number; that is, it is not the root of any polynomial with rational number coefficients.
NonEuclidean geometry
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:
Two squares can tile the sphere with 2 squares around each vertex and 180degree . Each square covers an entire hemisphere and their vertices lie along a great circle. This is called a spherical square dihedron. The Schläfli symbol is {4,2}.  Six squares can tile the sphere with 3 squares around each vertex and 120degree . This is called a spherical cube. The Schläfli symbol is {4,3}.  Square tiling the Euclidean plane with 4 around each vertex, with each square having an internal angle of 90°. The Schläfli symbol is Square tiling. 
Squares can tile the Hyperbolic space 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. 
Crossed square
A
crossed square is a
faceting of the square, a selfintersecting polygon created by removing two opposite edges of a square and reconnecting by its two diagonals. It has half the symmetry of the square, Dih
_{2}, order 4. It has the same vertex arrangement as the square, and is vertextransitive. It appears as two 454590 triangle with a common vertex, but the geometric intersection is not considered a vertex.
A crossed square is sometimes likened to a bow tie or butterfly. the crossed rectangle is related, as a faceting of the rectangle, both special cases of crossed quadrilaterals.
The interior of a crossed square can have a polygon density of ±1 in each triangle, dependent upon the winding orientation as clockwise or counterclockwise.
A square and a crossed square have the following properties in common:

Opposite sides are equal in length.

The two diagonals are equal in length.

It has two lines of reflectional symmetry and rotational symmetry of order 2 (through 180°).
It exists in the vertex figure of a uniform star polyhedra, the tetrahemihexahedron.
Graphs
The K
_{4} complete graph is often drawn as a square with all 6 possible edges connected, hence appearing as a square with both diagonals drawn. This graph also represents an orthographic projection of the 4 vertices and 6 edges of the regular 3
simplex (
tetrahedron).
See also
External links