A line is a collection of points along a straight path extending indefinitely in both directions. This is indicated by the arrows at each end, shown in the figure below.

A line has one dimension, its length. Two non-overlapping points determine a unique line and we can name the line with those two points or any other two points on the line.

line AB or BA defined by points A and B

Lines are used in shapes, angles, and many other geometric contexts.

Lines and angles

When two non-overlapping lines intersect at a point, two pairs of vertical angles are formed.

When 2 non-overlapping lines never intersect, they are called parallel lines.

When 2 lines intersect at a right angle, we say the lines are perpendicular to each other.

Lines and planes

Lines and polygons

Three or more intersecting lines can form a polygon whose vertices are the points at which the lines intersect.

Line l, m, n above intersect at points P, Q, and R forming triangle PQR. Also, P, Q, R are the vertices for the triangle formed.

Determination of planes

A plane can be determined in a number of ways:

Properties of planes