# Line segment

The geometric definition of a line segment
historical image – create a line segment (1699)

In geometry, 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. Examples of line segments include the sides of a triangle or square. More generally, when the end points are both vertices of a polygon, the line segment is either an edge (of that polygon) if they are adjacent vertices, or otherwise a diagonal. When the end points both lie on a curve such as a circle, a line segment is called a chord (of that curve).

## In real or complex vector spaces

If V is a vector space over $\mathbb{R}$ or $\mathbb{C}$, and L is a subset of V, then L is a line segment if L can be parameterized as

$L = \{ \mathbf{u}+t\mathbf{v} \mid t\in[0,1]\}$

for some vectors $\mathbf{u}, \mathbf{v} \in V\,\!$, in which case the vectors u and u + v are called the end points of L.

Sometimes one needs to distinguish between "open" and "closed" line segments. Then one defines a closed line segment as above, and an open line segment as a subset L that can be parametrized as

$L = \{ \mathbf{u}+t\mathbf{v} \mid t\in(0,1)\}$

for some vectors $\mathbf{u}, \mathbf{v} \in V\,\!$.

Equivalently, a line segment is the convex hull of two points. Thus, the line segment can be expressed as a convex combination of the segment's two end points.

In geometry, it is sometimes defined that a point B is between two other points A and C, if the distance AB added to the distance BC is equal to the distance AC. Thus in $\mathbb{R}^2$ the line segment with endpoints A = (ax, ay) and C = (cx, cy) is the following collection of points:

$\{ (x,y) | \sqrt{(x-c_x)^2 + (y-c_y)^2} + \sqrt{(x-a_x)^2 + (y-a_y)^2} = \sqrt{(c_x-a_x)^2 + (c_y-a_y)^2}\}$.

## In proofs

In an axiomatic treatment of geometry, the notion of betweenness is either assumed to satisfy a certain number of axioms, or else be defined in terms of an isometry of a line (used as a coordinate system).

Segments play an important role in other theories. For example, a set is convex if the segment that joins any two points of the set is contained in the set. This is important because it transforms some of the analysis of convex sets to the analysis of a line segment. The Segment Addition Postulate can be used to add congruent segment or segments with equal lengths and consequently substitute other segments into another statement to make segments congruent.

## As a degenerate ellipse

A line segment can be viewed as a degenerate case of an ellipse in which the semiminor axis goes to zero, the foci go to the endpoints, and the eccentricity goes to one. As a degenerate orbit this is a radial elliptic trajectory.