A geometrical object that is straight, infinitely long and infinitely thin.

Try this
Drag the orange dot at P or Q and see how the line PQ behaves.

In the figure above, the line PQ passes through the points P and Q, and goes off in both directions forever, and is perfectly straight. A line, strictly speaking, has no ends.

A line is one-dimensional. It has zero width. If you draw a line with a pencil, examination with a microscope would show that the pencil mark has a measurable width. The pencil line is just a way to illustrate the idea on paper. In geometry however, a line has no width.

A straight line is the shortest distance between any two points on a plane.

Lines are commonly named in two ways:

- By any two points on the line.

In the figure above, the line would be called JK because it passes through the two points J and K. Recall that points are usually labelled with single upper-case (capital) letters. There is a shortcut way of writing this: This is read as "line JK". The two arrow heads indicate that this is a line which passes through J and K but goes on forever in both directions. - By a single letter.

The line above could also be called simply "y". By convention, this is usually a single lower case (small) letter. This method is sometimes used when the line does not have two points on it to define it.

If a line is not straight, we usually refer to it as a curve or arc. In plane geometry the word 'line' is usually taken to mean a straight line.

If a set of points are lined up in such a way that a line can be drawn through all of them, the points are said to be collinear. See Collinear definition.

In another branch of mathematics called coordinate geometry,
the points that define a line are located on the plane using their
coordinates - two numbers that show where the point is positioned.

For more on this, see
Definition of a line (Coordinate Geometry).

- Point definition
- Line definition
- Vertical lines
- Horizontal lines
- Line segment
- Midpoint of a line
- Ray
- Angle
- Opposite rays
- Intersection
- Parallel lines
- Transversal
- Line bisector
- Perpendicular bisector
- Coplanar
- Collinear points
- Plane

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