In geometry, a line is written as two letters with a double-headed arrow overtop:
In plain language, this means “the line through the points and ”. The arrowheads are part of the notation: a line has no ends, so and are simply two points it passes through, not where it starts and stops.
A bar instead of an arrow means the line segment: the finite piece of the line between the two points, including both of them.
This is the notation used on the parallel lines and perpendicular lines pages, where the figures show segments rather than full lines.
The symbol for two perpendicular lines is a horizontal line with another line drawn perpendicular to it.
Parallel lines are denoted by the parallel symbol placed between the notation of the two lines. A line is denoted by the start and end letter with a line over top.