geometry line

Horizontal

A horizontal line is one which runs left-to-right across the page.

In geometry, a horizontal line is one which runs from left to right across the page. It comes from the word 'horizon', in the sense that horizontal lines are parallel to the horizon.

The horizon is horizontal

Its cousin is the vertical line which runs up and down the page. A vertical line is perpendicular to a horizontal line. (See perpendicular lines).

Plane

From Latin: plantum - "flat surface,"

A flat surface that is infinitely large and with zero thickness

Clearly, when you read the above definition, such a thing cannot possibly really exist. Imagine a flat sheet of metal. Now make it infinitely large in both directions. This means that no matter how far you go, you never reach its edges. Now imagine that it is so thin that it actually has no thickness at all. In spite of this, it remains completely rigid and flat. This is the 'plane' in geometry.

It fits into a scheme that starts with a point, which has no dimensions and goes up through solids which have three dimensions:

point	line	Plane	Solid
Zero dimensions	One dimension	Two dimensions	Three dimensions

It is difficult to draw planes, since the edges have to be drawn. When you see a picture that represents a plane, always remember that it actually has no edges, and it is infinitely large.

The plane has two dimensions: length and width. But since the plane is infinitely large, the length and width cannot be measured.

Just as a line is defined by two points, a plane is defined by three points. Given three points that are not collinear, there is just one plane that contains all three.

Parallel planes

You can think of parallel planes as sheets of cardboard one above the other with a gap between them. Parallel planes are the same distance apart everywhere, and so they never touch.

Intersecting planes

If two planes are not parallel, then they will intersect (cross over) each other somewhere. Two planes alwaysintersect at a line, as shown on the right.

This is similar to the way two lines intersect at apoint.

Naming of planes

Planes are usually named with a single upper case (capital) letter in a cursive script such as

Coordinate Geometry

In another branch of mathematics called coordinate geometry, points are located on the plane using their coordinates - two numbers that show where the point is positioned. To achieve this, the plane is thought to have two scales at right angles. Using a pair of numbers, any point on the plane can be uniquely described.