# Definition:Parallel (Geometry)/Lines

## Definition

In the words of Euclid:

**Parallel**straight lines are straight lines which, being in the same plane and being produced indefinitely in either direction, do not meet one another in either direction.

(*The Elements*: Book $\text{I}$: Definition $23$)

The contemporary interpretation of the concept of parallelism declares that a straight line is parallel to itself.

## Comment

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Different geometries allow different conditions for the existence of parallel lines.

- Euclidean geometry allows that exactly
*one*line through a given point can be constructed parallel to a given line; - Hyperbolic geometry allows for an infinite number of such;
- Elliptical geometry does
*not*allow construction of such lines.

## Reflexivity

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An attempt can be made to define parallelism by suggesting that the perpendiculars dropped from one (line or plane) to another (line or plane) are the same length everywhere along the line or plane, but this interpretation does not work in the context of non-Euclidean geometries, and is in fact no more than a derivable consequence of the definition of parallel as given here.

## Also see

- Results about
**parallel lines**can be found**here**.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**parallel** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**parallel**