 If a vertical line intersects a curve on an "xy"-plane more than once then for one value of "x" the curve has more than one value of "y", and so, the curve does not represent a function.If all vertical lines intersect a curve at most once then the curve represents a function.To use the vertical line test, take a ruler or other straight edge and draw a line parallel to the "y"-axis for any chosen value of "x".