# Geometry Help: Segments, Rays, Parallel Lines, and Planes

## Discovering, line segments, rays, parallel lines, planes, and more.

### Definition: Opposite Rays

Opposite rays are collinear rays with the same endpoint.

Basically, collinear means “on the same line”, so the two rays will look like one line.

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### Definition: Parallel Lines

Parallel lines are coplanar lines, meaning “on the same plane”, that don’t intersect.

Source

The sign for parallelism is a sideways equal sign.

### Definition: Skew Lines

Noncoplanar lines. Very simple; noncoplanar, meaning “not within the same plane”.

Source

Lines PS and RY are noncoplanar; therefore, they are skew.

### Naming Segments and Rays

#### Naming Segments

When naming a segment, name both endpoints. The order of the two points written does not matter. The symbol shown above the two points is a short line.

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#### Naming Rays

When naming a ray, name the endpoint first, then the second point.

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The ray above is called a right ray, because it faces to the right. Left rays, like the one below, face to the left. When naming a ray, the symbol is an arrow pointing to the right. It always points to the right, no matter if it’s a right ray or a left ray. In a left ray, make sure you name the ENDPOINT first. If you do not, and name the points from left to right, IT WILL NOT BE CORRECT!

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