Vertical and Horizontal Lines

Horizontal and vertical lines form the grid that helps us locate points on a coordinate graph.

Think about a soccer field. Before a match begins, groundskeepers paint markings so players and referees can identify the playing area. Two such markings are the touch lines along the sides and the goal lines at each end.

Suppose we place the field on a coordinate graph with its bottom-left corner at the origin. Its length lies along the x-axis, and its width lies along the y-axis.

The touch lines run from left to right. On the graph, these are horizontal lines. As you move along a touch line, the x-coordinate changes, but the y-coordinate stays the same. Since there is no vertical change, the slope is zero. Its equation is y equals a constant.

The goal lines run up and down. On the graph, these are vertical lines. As you move along a goal line, the y-coordinate changes, but the x-coordinate stays the same. Since there is no horizontal change, the slope is undefined. Its equation is x equals a constant.

Just as the field markings define the playing area, these lines help us locate points on a coordinate graph.