Q:
# What is the difference between vertical and horizontal lines?

Horizontal lines are parallel to the horizon or parallel to level ground. They have a slope of zero and are parallel to the x-axis on a graph. Vertical lines are perpendicular to the horizon, parallel to the y-axis on a graph and have undefined slope.

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The term "horizontal" derives from the Latin "horizontem" which means "flat." The term "vertical" derives from the Latin "vertex," which means "highest point."

The slope of a line refers to the change in the rise, or y-value, divided by the run, or the change in x-value. Because a horizontal line does not change in y-value with changes in x-value, its slope may be calculated as zero divided by any number. This results in a slope of zero. For vertical lines, the slope equation leads to a division by zero, which is undefined.

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