Line (geometry)
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Geometry 

Oxyrhynchus papyrus (P.Oxy. I 29) showing fragment of Euclid's Elements

History of geometry 
Branches
Euclidean geometry · NonEuclidean geometry · Analytic geometry · Riemannian geometry · Differential geometry · Projective geometry· Algebraic geometry

Research areas

Important concepts
Point · Line · Perpendicular · Parallel · Line segment · Ray · Plane · Length · Area · Volume · Vertex · Angle · Congruence · Similarity · Polygon · Triangle · Altitude · Hypotenuse · Pythagorean theorem · Quadrilateral · Trapezoid · Kite · Parallelogram ( Rhomboid, Rectangle, Rhombus, Square) · Diagonal · Symmetry · Curve · Circle · Area of a disk · Circumference · Diameter · Cylinder · Sphere · Pyramid · Dimensions ( one, two, three, four)

Geometers
Aryabhata · Ahmes · Apolonius · Archimedes · Baudhayana · Bolyai · Brahmagupta · Euclid · Pythagoras · Khayyám · Descartes · Pascal · Euler · Gauss · Ibn alYasamin · Jyeṣṭhadeva · Kātyāyana · Lobachevsky · Manava · Minggatu · Riemann · Klein · Parameshvara · Poincaré · Sijzi · Hilbert · Minkowski · Cartan · Veblen · Sakabe Kōhan · Gromov · Atiyah · Virasena · Yang Hui · Yasuaki Aida · Zhang Heng

The notion of line or straight line was introduced by ancient mathematicians to represent straight objects with negligible width and depth. Lines are an idealization of such objects. Thus, until seventeenth century, lines were defined like this: "The line is the first species of quantity, which has only one dimension, namely length, without any width nor depth, and is nothing else than the flow or run of the point which [...] will leave from its imaginary moving some vestige in length, exempt of any width. [...] The straight line is that which is equally extended between its points"
Euclid described a line as "breadthless length", and introduced several postulates as basic unprovable properties from which he constructed the geometry, which is now called Euclidean geometry to avoid confusion with other geometries which have been introduced since the end of nineteenth century (such as nonEuclidean geometry, projective geometry, and affine geometry).
In modern mathematics, given the multitude of geometries, the concept of a line is closely tied to the way the geometry is described. For instance, in analytic geometry, a line in the plane is often defined as the set of points whose coordinates satisfy a given linear equation, but in a more abstract setting, such as incidence geometry, a line may be an independent object, distinct from the set of points which lie on it.
When a geometry is described by a set of axioms, the notion of a line is usually left undefined (a socalled primitive object). The properties of lines are then determined by the axioms which refer to them. One advantage to this approach is the flexibility it gives to users of the geometry. Thus in differential geometry a line may be interpreted as a geodesic (shortest path between points), while in some projective geometries a line is a 2dimensional vector space (all linear combinations of two independent vectors). This flexibility also extends beyond mathematics and, for example, permits physicists to think of the path of a light ray as being a line.
A line segment is a part of a line that is bounded by two distinct end points and contains every point on the line between its end points. Depending on how the line segment is defined, either of the two end points may or may not be part of the line segment. Two or more line segments may have some of the same relationships as lines, such as being parallel, intersecting, or skew.
Definitions versus descriptions
All definitions are ultimately circular in nature since they depend on concepts which must themselves have definitions, a dependence which can not be continued indefinitely without returning to the starting point. To avoid this vicious circle certain concepts must be taken as primitive concepts; terms which are given no definition. In geometry, it is frequently the case that the concept of line is taken as a primitive. In those situations where a line is a defined concept, as in coordinate geometry, some other fundamental ideas are taken as primitives. When the line concept is a primitive, the behaviour and properties of lines are dictated by the axioms which they must satisfy.
In a nonaxiomatic or simplified axiomatic treatment of geometry, the concept of a primitive notion may be too abstract to be dealt with. In this circumstance it is possible that a description or mental image of a primitive notion is provided to give a foundation to build the notion on which would formally be based on the (unstated) axioms. Descriptions of this type may be referred to, by some authors, as definitions in this informal style of presentation. These are not true definitions and could not be used in formal proofs of statements. The "definition" of line in Euclid's Elements falls into this category. Even in the case where a specific geometry is being considered (for example, Euclidean geometry), there is no generally accepted agreement amongst authors as to what an informal description of a line should be when the subject is not being treated formally.
Euclidean geometry
When geometry was first formalised by Euclid in the Elements, he defined a line to be "breadthless length" with a straight line being a line "which lies evenly with the points on itself". These definitions serve little purpose since they use terms which are not, themselves, defined. In fact, Euclid did not use these definitions in this work and probably included them just to make it clear to the reader what was being discussed. In modern geometry, a line is simply taken as an undefined object with properties given by axioms, but is sometimes defined as a set of points obeying a linear relationship when some other fundamental concept is left undefined.
In an axiomatic formulation of Euclidean geometry, such as that of Hilbert (Euclid's original axioms contained various flaws which have been corrected by modern mathematicians), a line is stated to have certain properties which relate it to other lines and points. For example, for any two distinct points, there is a unique line containing them, and any two distinct lines intersect in at most one point. In two dimensions, i.e., the Euclidean plane, two lines which do not intersect are called parallel. In higher dimensions, two lines that do not intersect may be parallel if they are contained in a plane, or skew if they are not.
Any collection of finitely many lines partitions the plane into convex polygons (possibly unbounded); this partition is known as an arrangement of lines.
Ray
Given a line and any point A on it, we may consider A as decomposing this line into two parts. Each such part is called a ray (or halfline) and the point A is called its initial point. The point A is considered to be a member of the ray. Intuitively, a ray consists of those points on a line passing through A and proceeding indefinitely, starting at A, in one direction only along the line. However, in order to use this concept of a ray in proofs a more precise definition is required.
Given distinct points A and B, they determine a unique ray with initial point A. As two points define a unique line, this ray consists of all the points between A and B (including A and B) and all the points C on the line through A and B such that B is between A and C. This is, at times, also expressed as the set of all points C such that A is not between B and C. A point D, on the line determined by A and B but not in the ray with initial point A determined by B, will determine another ray with initial point A. With respect to the AB ray, the AD ray is called the opposite ray.
Thus, we would say that two different points, A and B, define a line and a decomposition of this line into the disjoint union of an open segment (A, B) and two rays, BC and AD (the point D is not drawn in the diagram, but is to the left of A on the line AB). These are not opposite rays since they have different initial points.
The definition of a ray depends upon the notion of betweenness for points on a line. It follows that rays exist only for geometries for which this notion exists, typically Euclidean geometry or affine geometry over an ordered field. On the other hand, rays do not exist in projective geometry nor in a geometry over a non ordered field, like the complex numbers or any finite field.
In topology, a ray in a space X is a continuous embedding R^{+} → X. It is used to define the important concept of end of the space.
Coordinate geometry
In coordinate geometry, lines in a Cartesian plane can be described algebraically by linear equations. In two dimensions, the equation for nonvertical lines is often given in the slopeintercept form:
where:
 m is the slope or gradient of the line.
 c is the yintercept of the line.
 x is the independent variable of the function y = f(x).
The slope of the line through points A(x_{a}, y_{a}) and B(x_{b}, y_{b}), when x_{a} ≠ x_{b}, is given by m = (y_{b}y_{a})/(x_{b}x_{a}) and the equation of this line can be written y = m(x  x_{a}) + y_{a}.
In R^{2}, every line L (including vertical lines) is described by a linear equation of the form
with fixed real coefficients a, b and c such that a and b are not both zero. Using this form, vertical lines correspond to the equations with b = 0.
There are many variant ways to write the equation of a line which can all be converted from one to another by algebraic manipulation. These forms (see Linear equation for other forms) are generally named by the type of information (data) about the line that is needed to write down the form. Some of the important data of a line is its slope, xintercept, known points on the line and yintercept.
The equation of the line passing through two different points and may be written as
 .
If x_{0} ≠ x_{1}, this equation may be rewritten as
or
In three dimensions, lines can not be described by a single linear equation, so they are frequently described by parametric equations:
where:
 x, y, and z are all functions of the independent variable t which ranges over the real numbers.
 (x_{0}, y_{0}, z_{0}) is any point on the line.
 a, b, and c are related to the slope of the line, such that the vector (a, b, c) is parallel to the line.
They may also be described as the simultaneous solutions of two linear equations
such that and are not proportional (the relations imply t = 0). This follows since in three dimensions a single linear equation typically describes a plane and a line is what is common to two distinct intersecting planes.
Vector equation
The vector equation of the line through points A and B is given by r = OA + λAB (where λ is a scalar).
If a is vector OA and b is vector OB, then the equation of the line can be written: r = a + λ(b  a).
A ray starting at point A is described by limiting λ. One ray is obtained if λ ≥ 0, and the opposite ray comes from λ ≤ 0.
Collinear points
Three points are said to be collinear if they lie on the same line. In Euclidean space, three points usually determine a plane, but in the case of three collinear points this does not happen.
In coordinate geometry, in ndimensional space the points X=(x_{1}, x_{2}, ..., x_{n}), Y=(y_{1}, y_{2}, ..., y_{n}), and Z=(z_{1}, z_{2}, ..., z_{n}) are collinear if the matrix
has a rank less than 3. In particular, for three points in the plane (n=2), above matrix is square and the points are collinear if and only if its determinant is zero.
When the distance d(a,b) between two points a and b is defined, the collinearity between three points may be expressed by:
 The points a, b and c are collinear if and only if d(x,a) = d(c,a) and d(x,b) = d(c,b) implies x=c.
In Euclidean geometry this property is true, since if c is not on the line determined by a and b there will be another point (not equal to c) which is just as far from a and b as the point c is (visualize the point on the other side of the line which is the mirror image of c).
In the geometries where the concept of a line is a primitive notion, as may be the case in some synthetic geometries, other methods of determining collinearity are needed.
Euclidean space
In Euclidean space, R^{n} (and analogously in every other affine space), the line L passing through two different points a and b (considered as vectors) is the subset
The direction of the line is from a (t = 0) to b (t = 1), or in other words, in the direction of the vector ba. Different choices of a and b can yield the same line.
Types of lines
In a sense, all lines in Euclidean geometry are equal, in that, without coordinates, one can not tell them apart from one another. However, lines may play special roles with respect to other objects in the geometry and be divided into types according to that relationship. For instance, with respect to a conic, lines can be:
 Tangent lines,
 Secant lines,
 Exterior lines, which do not meet the conic at any point of the Euclidean plane, or a more specialized
 directrix.
For more general algebraic curves, lines could also be:
 isecant lines, meeting the curve in i points counted without multiplicity, or
 asymptotes.
With respect to triangles we have:
 the Euler line, and
 the Simson lines.
For a hexagon with vertices lying on a conic we have the Pascal line and, in the special case where the conic is a pair of lines, we have the Pappus line.
Projective geometry
In many models of projective geometry, the representation of a line rarely conforms to the notion of the "straight curve" as it is visualised in Euclidean geometry. In Elliptic geometry we see a typical example of this. In the spherical representation of elliptic geometry, lines are represented by great circles of a sphere with diametrically opposite points identified. In a different model of elliptic geometry, lines are represented by Euclidean planes passing through the origin. Even though these representations are visually distinct, they satisfy all the properties (such as, two points determining a unique line) that make them suitable representations for lines in this geometry.
Geodesics
The "straightness" of a line, interpreted as the property that it minimizes distances between its points, can be generalized and leads to the concept of geodesics in metric spaces.