It's a horizontal line! . the graph of y = -2 is. So, what's the slope of this line? Grab two points and see? m = ( y2 - y1 ) / ( x2 - x1 ) = -2. The slope of a horizontal . Mathwords: Undefined Slope. The 'slope' of a vertical line. A vertical line has undefined slope because all points on the line have the same x-coordinate. As a result the formula used for slope has a denominator of 0, which makes the slope undefined It is a vertical line through 2 on the - axis. Vertical Lines A vertical line is a line whose equation is of the form =ℎ, where ℎ is a real number. All points that lie on the line have an -coordinate of ℎ. The slope of any vertical line is undefined. Aug 1, 2016 - results in the bogeyman of mathematics, division by zero. That's why. However, in some cases we define the slope of a vertical line as ∞ . Jan 16, 2010 - positive slope. Note that when a line has a positive slope it rises up left to right. . Note that when the line is vertical the slope is undefined. Since anything divided by 0 is undefined, the slope of a vertical line is undefined. An easy way to remember the difference is that the word horizontal has a 'z' in . Definition: A line on the coordinate plane where all points on the line have the same x-coordinate. . Or put another way, for a vertical line the slope is undefined. In mathematics, the slope or gradient of a line is a number that describes both the direction and . If a line is vertical the slope is undefined (see below). The rise . Thus, a 45° rising line has a slope of +1 and a 45° falling line has a slope of −1.