What is the equation of a verticle and horizontal line ?

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Think of a coordinate system where there are two axes usually refered to x for the horizontal and y for the vertical. If we set a constant number, say x = 5 then that is a vertical line since it is only parallel with the x-axis at one point as for the y-axis it is parallel for all points. A vertical line is simply a constant y and variable x since say y = 3 is parallel with the y-axis at one point but parallel with the x-axis for all points.