Line

A line is a straight one-dimensional figure having no thickness and extending infinitely in both directions. A line is sometimes called a straight line or, more archaically, a right line (Casey 1893), to emphasize that it has no "wiggles" anywhere along its length. While lines are intrinsically one-dimensional objects, they may be embedded in higher dimensional spaces.

Harary (1994) called an edge of a graph a "line."

A line is uniquely determined by two points, and the line passing through points and is denoted . Similarly, the length of the finite line segment terminating at these points may be denoted . A line may also be denoted with a single lower-case letter (Jurgensen et al. 1963, p. 22).

Euclid defined a line as a "breadthless length," and a straight line as a line that "lies evenly with the points on itself" (Kline 1956, Dunham 1990).

Consider first lines in a two-dimensional plane. Two lines lying in the same plane that do not intersect one another are said to be parallel lines. Two lines lying in different planes that do not intersect one another are said to be skew lines.

The line with x-intercept and y-intercept is given by the intercept form

(1)

It is not uncommon for lines in intercept form to be rewritten in what's known as standard form:

(2)

The line through with slope is given by the point-slope form

(3)

The line with -intercept and slope is given by the slope-intercept form

(4)

The line through and is given by the two-point form

(5)

A parametric form is given by

			(6)
			(7)

Other forms are

(8)

(9)

(10)

A line in two dimensions can also be represented as a vector. The vector along the line

(11)

is given by

(12)

where . Similarly, vectors of the form

(13)

are perpendicular to the line.

Three points lie on a line if

(14)

The angle between lines

			(15)
			(16)

is

(17)

The line joining points with trilinear coordinates and is the set of point satisfying

(18)

(19)

The line through in the direction and the line through in direction intersect iff

(20)

The line through a point parallel to

(21)

is

(22)

The lines

			(23)
			(24)

are parallel if

(25)

for all , and perpendicular if

(26)

for all (Sommerville 1961, Kimberling 1998, p. 29).

The line through a point perpendicular to (◇) is given by

(27)

In three-dimensional space, the line passing through the point and parallel to the nonzero vector has parametric equations

			(28)
			(29)
			(30)

written concisely as

(31)

Similarly, the line in three dimensions passing through and has parametric vector equation

(32)

where this parametrization corresponds to and .