Varsity Tutors
Varsity Tutors: Hotmath: Algebra Review: Horizontal Lines
Find a quick, concise explanation of equations of horizontal lines. An example is given and clearly explained for equations of both horizontal and vertical lines.
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Equation of a Line Parallel to an Axis
Dr. Math gives a short discussion on how to find the equation of a line parallel to an axis (, parallel or horizontal.)