In geometry, a **polygon** ( /ˈpɒlɪɡɒn/ ) is a plane figure that is described by a finite number of straight line segments connected to form a closed polygonal chain or *polygonal circuit*. The solid plane region, the bounding circuit, or the two together, may be called a polygon.

- Etymology
- Classification
- Number of sides
- Convexity and non-convexity
- Equality and symmetry
- Miscellaneous
- Properties and formulas
- Angles
- Area
- Centroid
- Generalizations
- Naming
- Constructing higher names
- History
- In nature
- Computer graphics
- See also
- References
- Bibliography
- Notes
- External links

The segments of a polygonal circuit are called its * edges * or *sides*, and the points where two edges meet are the polygon's * vertices * (singular: vertex) or *corners*. The interior of a solid polygon is sometimes called its *body*. An ** n-gon** is a polygon with

A simple polygon is one which does not intersect itself. Mathematicians are often concerned only with the bounding polygonal chains of simple polygons and they often define a polygon accordingly. A polygonal boundary may be allowed to cross over itself, creating star polygons and other self-intersecting polygons.

A polygon is a 2-dimensional example of the more general polytope in any number of dimensions. There are many more generalizations of polygons defined for different purposes.

The word *polygon* derives from the Greek adjective πολύς (*polús*) 'much', 'many' and γωνία (*gōnía*) 'corner' or 'angle'. It has been suggested that γόνυ (*gónu*) 'knee' may be the origin of *gon*.^{ [1] }

Polygons are primarily classified by the number of sides. See the table below.

Polygons may be characterized by their convexity or type of non-convexity:

- Convex: any line drawn through the polygon (and not tangent to an edge or corner) meets its boundary exactly twice. As a consequence, all its interior angles are less than 180°. Equivalently, any line segment with endpoints on the boundary passes through only interior points between its endpoints.
- Non-convex: a line may be found which meets its boundary more than twice. Equivalently, there exists a line segment between two boundary points that passes outside the polygon.
- Simple: the boundary of the polygon does not cross itself. All convex polygons are simple.
- Concave: Non-convex and simple. There is at least one interior angle greater than 180°.
- Star-shaped: the whole interior is visible from at least one point, without crossing any edge. The polygon must be simple, and may be convex or concave. All convex polygons are star-shaped.
- Self-intersecting: the boundary of the polygon crosses itself. The term
*complex*is sometimes used in contrast to*simple*, but this usage risks confusion with the idea of a*complex polygon*as one which exists in the complex Hilbert plane consisting of two complex dimensions. - Star polygon: a polygon which self-intersects in a regular way. A polygon cannot be both a star and star-shaped.

- Equiangular: all corner angles are equal.
- Cyclic: all corners lie on a single circle, called the circumcircle.
- Isogonal or vertex-transitive: all corners lie within the same symmetry orbit. The polygon is also cyclic and equiangular.
- Equilateral: all edges are of the same length. The polygon need not be convex.
- Tangential: all sides are tangent to an inscribed circle.
- Isotoxal or edge-transitive: all sides lie within the same symmetry orbit. The polygon is also equilateral and tangential.
- Regular: the polygon is both
*isogonal*and*isotoxal*. Equivalently, it is both*cyclic*and*equilateral*, or both*equilateral*and*equiangular*. A non-convex regular polygon is called a*regular star polygon*.

- Rectilinear: the polygon's sides meet at right angles, i.e., all its interior angles are 90 or 270 degrees.
- Monotone with respect to a given line
*L*: every line orthogonal to L intersects the polygon not more than twice.

Euclidean geometry is assumed throughout.

Any polygon has as many corners as it has sides. Each corner has several angles. The two most important ones are:

**Interior angle**– The sum of the interior angles of a simple*n*-gon is (*n*− 2)π radians or (*n*− 2) × 180 degrees. This is because any simple*n*-gon ( having*n*sides ) can be considered to be made up of (*n*− 2) triangles, each of which has an angle sum of π radians or 180 degrees. The measure of any interior angle of a convex regular*n*-gon is radians or degrees. The interior angles of regular star polygons were first studied by Poinsot, in the same paper in which he describes the four regular star polyhedra: for a regular -gon (a*p*-gon with central density*q*), each interior angle is radians or degrees.^{ [2] }**Exterior angle**– The exterior angle is the supplementary angle to the interior angle. Tracing around a convex*n*-gon, the angle "turned" at a corner is the exterior or external angle. Tracing all the way around the polygon makes one full turn, so the sum of the exterior angles must be 360°. This argument can be generalized to concave simple polygons, if external angles that turn in the opposite direction are subtracted from the total turned. Tracing around an*n*-gon in general, the sum of the exterior angles (the total amount one rotates at the vertices) can be any integer multiple*d*of 360°, e.g. 720° for a pentagram and 0° for an angular "eight" or antiparallelogram, where*d*is the density or starriness of the polygon. See also orbit (dynamics).

In this section, the vertices of the polygon under consideration are taken to be in order. For convenience in some formulas, the notation (*x _{n}*,

If the polygon is non-self-intersecting (that is, simple), the signed area is

or, using determinants

where is the squared distance between and ^{ [3] }^{ [4] }

The signed area depends on the ordering of the vertices and of the orientation of the plane. Commonly, the positive orientation is defined by the (counterclockwise) rotation that maps the positive x-axis to the positive y-axis. If the vertices are ordered counterclockwise (that is, according to positive orientation), the signed area is positive; otherwise, it is negative. In either case, the area formula is correct in absolute value. This is commonly called the shoelace formula or Surveyor's formula.^{ [5] }

The area *A* of a simple polygon can also be computed if the lengths of the sides, *a*_{1}, *a*_{2}, ..., *a _{n}* and the exterior angles,

The formula was described by Lopshits in 1963.^{ [6] }

If the polygon can be drawn on an equally spaced grid such that all its vertices are grid points, Pick's theorem gives a simple formula for the polygon's area based on the numbers of interior and boundary grid points: the former number plus one-half the latter number, minus 1.

In every polygon with perimeter *p* and area *A *, the isoperimetric inequality holds.^{ [7] }

For any two simple polygons of equal area, the Bolyai–Gerwien theorem asserts that the first can be cut into polygonal pieces which can be reassembled to form the second polygon.

The lengths of the sides of a polygon do not in general determine its area.^{ [8] } However, if the polygon is cyclic then the sides *do* determine the area.^{ [9] } Of all *n*-gons with given side lengths, the one with the largest area is cyclic. Of all *n*-gons with a given perimeter, the one with the largest area is regular (and therefore cyclic).^{ [10] }

Many specialized formulas apply to the areas of regular polygons.

The area of a regular polygon is given in terms of the radius *r* of its inscribed circle and its perimeter *p* by

This radius is also termed its apothem and is often represented as *a*.

The area of a regular *n*-gon in terms of the radius *R* of its circumscribed circle can be expressed trigonometrically as:^{ [11] }^{ [12] }

The area of a regular *n*-gon inscribed in a unit-radius circle, with side *s* and interior angle can also be expressed trigonometrically as:

The area of a self-intersecting polygon can be defined in two different ways, giving different answers:

- Using the formulas for simple polygons, we allow that particular regions within the polygon may have their area multiplied by a factor which we call the
*density*of the region. For example, the central convex pentagon in the center of a pentagram has density 2. The two triangular regions of a cross-quadrilateral (like a figure 8) have opposite-signed densities, and adding their areas together can give a total area of zero for the whole figure.^{ [13] } - Considering the enclosed regions as point sets, we can find the area of the enclosed point set. This corresponds to the area of the plane covered by the polygon or to the area of one or more simple polygons having the same outline as the self-intersecting one. In the case of the cross-quadrilateral, it is treated as two simple triangles.
^{[ citation needed ]}

Using the same convention for vertex coordinates as in the previous section, the coordinates of the centroid of a solid simple polygon are

In these formulas, the signed value of area must be used.

For triangles (*n* = 3), the centroids of the vertices and of the solid shape are the same, but, in general, this is not true for *n* > 3. The centroid of the vertex set of a polygon with n vertices has the coordinates

The idea of a polygon has been generalized in various ways. Some of the more important include:

- A spherical polygon is a circuit of arcs of great circles (sides) and vertices on the surface of a sphere. It allows the digon, a polygon having only two sides and two corners, which is impossible in a flat plane. Spherical polygons play an important role in cartography (map making) and in Wythoff's construction of the uniform polyhedra.
- A skew polygon does not lie in a flat plane, but zigzags in three (or more) dimensions. The Petrie polygons of the regular polytopes are well known examples.
- An apeirogon is an infinite sequence of sides and angles, which is not closed but has no ends because it extends indefinitely in both directions.
- A skew apeirogon is an infinite sequence of sides and angles that do not lie in a flat plane.
- A complex polygon is a configuration analogous to an ordinary polygon, which exists in the complex plane of two real and two imaginary dimensions.
- An abstract polygon is an algebraic partially ordered set representing the various elements (sides, vertices, etc.) and their connectivity. A real geometric polygon is said to be a
*realization*of the associated abstract polygon. Depending on the mapping, all the generalizations described here can be realized. - A polyhedron is a three-dimensional solid bounded by flat polygonal faces, analogous to a polygon in two dimensions. The corresponding shapes in four or higher dimensions are called polytopes.
^{ [14] }(In other conventions, the words*polyhedron*and*polytope*are used in any dimension, with the distinction between the two that a polytope is necessarily bounded.^{ [15] })

The word *polygon* comes from Late Latin *polygōnum* (a noun), from Greek πολύγωνον (*polygōnon/polugōnon*), noun use of neuter of πολύγωνος (*polygōnos/polugōnos*, the masculine adjective), meaning "many-angled". Individual polygons are named (and sometimes classified) according to the number of sides, combining a Greek-derived numerical prefix with the suffix *-gon*, e.g. * pentagon *, * dodecagon *. The triangle, quadrilateral and nonagon are exceptions.

Beyond decagons (10-sided) and dodecagons (12-sided), mathematicians generally use numerical notation, for example 17-gon and 257-gon.^{ [16] }

Exceptions exist for side counts that are more easily expressed in verbal form (e.g. 20 and 30), or are used by non-mathematicians. Some special polygons also have their own names; for example the regular star pentagon is also known as the pentagram.

Name | Sides | Properties |
---|---|---|

monogon | 1 | Not generally recognised as a polygon,^{ [17] } although some disciplines such as graph theory sometimes use the term.^{ [18] } |

digon | 2 | Not generally recognised as a polygon in the Euclidean plane, although it can exist as a spherical polygon.^{ [19] } |

triangle (or trigon) | 3 | The simplest polygon which can exist in the Euclidean plane. Can tile the plane. |

quadrilateral (or tetragon) | 4 | The simplest polygon which can cross itself; the simplest polygon which can be concave; the simplest polygon which can be non-cyclic. Can tile the plane. |

pentagon | 5 | ^{ [20] } The simplest polygon which can exist as a regular star. A star pentagon is known as a pentagram or pentacle. |

hexagon | 6 | ^{ [20] } Can tile the plane. |

heptagon (or septagon) | 7 | ^{ [20] } The simplest polygon such that the regular form is not constructible with compass and straightedge. However, it can be constructed using a Neusis construction. |

octagon | 8 | ^{ [20] } |

nonagon (or enneagon) | 9 | ^{ [20] }"Nonagon" mixes Latin [novem = 9] with Greek; "enneagon" is pure Greek. |

decagon | 10 | ^{ [20] } |

hendecagon (or undecagon) | 11 | ^{ [20] } The simplest polygon such that the regular form cannot be constructed with compass, straightedge, and angle trisector. |

dodecagon (or duodecagon) | 12 | ^{ [20] } |

tridecagon (or triskaidecagon) | 13 | ^{ [20] } |

tetradecagon (or tetrakaidecagon) | 14 | ^{ [20] } |

pentadecagon (or pentakaidecagon) | 15 | ^{ [20] } |

hexadecagon (or hexakaidecagon) | 16 | ^{ [20] } |

heptadecagon (or heptakaidecagon) | 17 | Constructible polygon ^{ [16] } |

octadecagon (or octakaidecagon) | 18 | ^{ [20] } |

enneadecagon (or enneakaidecagon) | 19 | ^{ [20] } |

icosagon | 20 | ^{ [20] } |

icositetragon (or icosikaitetragon) | 24 | ^{ [20] } |

triacontagon | 30 | ^{ [20] } |

tetracontagon (or tessaracontagon) | 40 | ^{ [20] }^{ [21] } |

pentacontagon (or pentecontagon) | 50 | ^{ [20] }^{ [21] } |

hexacontagon (or hexecontagon) | 60 | ^{ [20] }^{ [21] } |

heptacontagon (or hebdomecontagon) | 70 | ^{ [20] }^{ [21] } |

octacontagon (or ogdoëcontagon) | 80 | ^{ [20] }^{ [21] } |

enneacontagon (or enenecontagon) | 90 | ^{ [20] }^{ [21] } |

hectogon (or hecatontagon)^{ [22] } | 100 | ^{ [20] } |

257-gon | 257 | Constructible polygon ^{ [16] } |

chiliagon | 1000 | Philosophers including René Descartes,^{ [23] } Immanuel Kant,^{ [24] } David Hume,^{ [25] } have used the chiliagon as an example in discussions. |

myriagon | 10,000 | Used as an example in some philosophical discussions, for example in Descartes's Meditations on First Philosophy |

65537-gon | 65,537 | Constructible polygon ^{ [16] } |

megagon ^{ [26] }^{ [27] }^{ [28] } | 1,000,000 | As with René Descartes's example of the chiliagon, the million-sided polygon has been used as an illustration of a well-defined concept that cannot be visualised.^{ [29] }^{ [30] }^{ [31] }^{ [32] }^{ [33] }^{ [34] }^{ [35] } The megagon is also used as an illustration of the convergence of regular polygons to a circle.^{ [36] } |

apeirogon | ∞ | A degenerate polygon of infinitely many sides. |

To construct the name of a polygon with more than 20 and less than 100 edges, combine the prefixes as follows.^{ [20] } The "kai" term applies to 13-gons and higher and was used by Kepler, and advocated by John H. Conway for clarity to concatenated prefix numbers in the naming of quasiregular polyhedra.^{ [22] }

Tens | and | Ones | final suffix | ||
---|---|---|---|---|---|

-kai- | 1 | -hena- | -gon | ||

20 | icosi- (icosa- when alone) | 2 | -di- | ||

30 | triaconta- (or triconta-) | 3 | -tri- | ||

40 | tetraconta- (or tessaraconta-) | 4 | -tetra- | ||

50 | pentaconta- (or penteconta-) | 5 | -penta- | ||

60 | hexaconta- (or hexeconta-) | 6 | -hexa- | ||

70 | heptaconta- (or hebdomeconta-) | 7 | -hepta- | ||

80 | octaconta- (or ogdoëconta-) | 8 | -octa- | ||

90 | enneaconta- (or eneneconta-) | 9 | -ennea- |

Polygons have been known since ancient times. The regular polygons were known to the ancient Greeks, with the pentagram, a non-convex regular polygon (star polygon), appearing as early as the 7th century B.C. on a krater by Aristophanes, found at Caere and now in the Capitoline Museum.^{ [37] }^{ [38] }

The first known systematic study of non-convex polygons in general was made by Thomas Bradwardine in the 14th century.^{ [39] }

In 1952, Geoffrey Colin Shephard generalized the idea of polygons to the complex plane, where each real dimension is accompanied by an imaginary one, to create complex polygons.^{ [40] }

Polygons appear in rock formations, most commonly as the flat facets of crystals, where the angles between the sides depend on the type of mineral from which the crystal is made.

Regular hexagons can occur when the cooling of lava forms areas of tightly packed columns of basalt, which may be seen at the Giant's Causeway in Northern Ireland, or at the Devil's Postpile in California.

In biology, the surface of the wax honeycomb made by bees is an array of hexagons, and the sides and base of each cell are also polygons.

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In computer graphics, a polygon is a primitive used in modelling and rendering. They are defined in a database, containing arrays of vertices (the coordinates of the geometrical vertices, as well as other attributes of the polygon, such as color, shading and texture), connectivity information, and materials.^{ [41] }^{ [42] }

Any surface is modelled as a tessellation called polygon mesh. If a square mesh has *n* + 1 points (vertices) per side, there are *n* squared squares in the mesh, or 2*n* squared triangles since there are two triangles in a square. There are (*n* + 1)^{2} / 2(*n*^{2}) vertices per triangle. Where *n* is large, this approaches one half. Or, each vertex inside the square mesh connects four edges (lines).

The imaging system calls up the structure of polygons needed for the scene to be created from the database. This is transferred to active memory and finally, to the display system (screen, TV monitors etc.) so that the scene can be viewed. During this process, the imaging system renders polygons in correct perspective ready for transmission of the processed data to the display system. Although polygons are two-dimensional, through the system computer they are placed in a visual scene in the correct three-dimensional orientation.

In computer graphics and computational geometry, it is often necessary to determine whether a given point *P* = (*x*_{0},*y*_{0}) lies inside a simple polygon given by a sequence of line segments. This is called the point in polygon test.^{ [43] }

In geometry, any polyhedron is associated with a second **dual** figure, where the vertices of one correspond to the faces of the other and the edges between pairs of vertices of one correspond to the edges between pairs of faces of the other. Such dual figures remain combinatorial or abstract polyhedra, but not all are also geometric polyhedra. Starting with any given polyhedron, the dual of its dual is the original polyhedron.

In geometry, a **regular icosahedron** is a convex polyhedron with 20 faces, 30 edges and 12 vertices. It is one of the five Platonic solids, and the one with the most faces.

In geometry, an **octahedron** is a polyhedron with eight faces, twelve edges, and six vertices. The term is most commonly used to refer to the **regular** octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.

In three-dimensional space, a **Platonic solid** is a regular, convex polyhedron. It is constructed by congruent, regular, polygonal faces with the same number of faces meeting at each vertex. Five solids meet these criteria:

A **quadrilateral** is a polygon in Euclidean plane geometry with four edges (sides) and four vertices (corners). Other names for quadrilateral include **quadrangle**, **tetragon**, and **4-gon**. A quadrilateral with vertices , , and is sometimes denoted as .

In geometry, a **tetrahedron**, also known as a **triangular pyramid**, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ordinary convex polyhedra and the only one that has fewer than 5 faces.

A **triangle** is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices *A*, *B*, and *C* is denoted .

In geometry, a **simplex** is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given space.

In geometry, a **solid angle** is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The point from which the object is viewed is called the *apex* of the solid angle, and the object is said to *subtend* its solid angle from that point.

In Euclidean geometry, a **regular polygon** is a polygon that is equiangular and equilateral. Regular polygons may be either **convex** or **star**. In the limit, a sequence of regular polygons with an increasing number of sides approximates a circle, if the perimeter or area is fixed, or a regular apeirogon, if the edge length is fixed.

A **central angle** is an angle whose apex (vertex) is the center O of a circle and whose legs (sides) are radii intersecting the circle in two distinct points A and B. Central angles are subtended by an arc between those two points, and the arc length is the central angle of a circle of radius one. The central angle is also known as the arc's angular distance.

In geometry, a **cross-polytope**, **hyperoctahedron**, **orthoplex**, or **cocube** is a regular, convex polytope that exists in *n*-dimensions. A 2-dimensional cross-polytope is a square, a 3-dimensional cross-polytope is a regular octahedron, and a 4-dimensional cross-polytope is a 16-cell. Its facets are simplexes of the previous dimension, while the cross-polytope's vertex figure is another cross-polytope from the previous dimension.

In Euclidean geometry, **Ptolemy's theorem** is a relation between the four sides and two diagonals of a cyclic quadrilateral. The theorem is named after the Greek astronomer and mathematician Ptolemy. Ptolemy used the theorem as an aid to creating his table of chords, a trigonometric table that he applied to astronomy.

In geometry, the area enclosed by a circle of radius r is π*r*^{2}. Here the Greek letter π represents the constant ratio of the circumference of any circle to its diameter, approximately equal to 3.14159.

In geometry, the **circumscribed circle** or **circumcircle** of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the **circumcenter** and its radius is called the **circumradius**.

In geometry, a **vertex**, often denoted by letters such as , , , , is a point where two or more curves, lines, or edges meet. As a consequence of this definition, the point where two lines meet to form an angle and the corners of polygons and polyhedra are vertices.

**Two-dimensional space** is a geometric setting in which two values are required to determine the position of an element. The set ℝ^{2} of pairs of real numbers with appropriate structure often serves as the canonical example of a two-dimensional Euclidean space. For a generalization of the concept, see dimension.

In geometry, a **pentagon** is any five-sided polygon or 5-gon. The sum of the internal angles in a simple pentagon is 540°.

- Coxeter, H.S.M.;
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- ↑ Schirra, Stefan (2008). "How Reliable Are Practical Point-in-Polygon Strategies?". In Halperin, Dan; Mehlhorn, Kurt (eds.).
*Algorithms - ESA 2008: 16th Annual European Symposium, Karlsruhe, Germany, September 15-17, 2008, Proceedings*. Lecture Notes in Computer Science.**5193**. Springer. pp. 744–755. doi:10.1007/978-3-540-87744-8_62.

Look up in Wiktionary, the free dictionary. polygon |

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