Horizontal and Vertical Lines — Cheat sheet

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The two families

Line	Slope	Equation
horizontal (parallel to $x$ -axis)	$m = 0$	$y = c$
vertical (parallel to $y$ -axis)	undefined	$x = c$

Watch for	Note
Which variable	the line is parallel to the axis not named in its equation
Vertical lines	no slope, so no $y = mx + n$ form exists
Two points sharing an abscissa	the line through them is vertical: $x$ equals that value
The axes themselves	the $x$ -axis is $y = 0$ ; the $y$ -axis is $x = 0$