When lines never
meet, no matter how far they are extended, they are said to be parallel.
To indicate when lines are parallel, small arrows are drawn on them.

For example,

The lines in an exercise book are parallel.

The horizontal lines
of mortar between brickwork are also parallel, as are railway lines.

Two parallel lines are always the same distance apart. In the same
way, two planes (flat surfaces) are parallel when they are the same
distance apart.

For example the ceiling and floor of a room are parallel.

Parallel lines must be located in the same plane, and parallel planes
must be located in the same three-dimensional space. A parallel
combination of a line and a plane may be located in the same three-dimensional
space. Lines parallel to each other have the same gradient.

Example of Parallel: