The more precise definition of the Vertical Line Test states: If a graph, G, in the xyplane has the property that there is at least one vertical line that contains more than one point on the graph, then there is no function y = f(x) whose graph is the graph G. This precision is only needed when one looks at functions where x and y are the only only independent and dependent variables. Other situations will be encountered in a PreCalculus or College Algebra course. Back to the functions page 
