Let be a real valued function defined in an open neighborhood of a real number . In classical geometry, the tangent line to the graph of the function at was the unique line through the point that did "not" meet the graph of transversally, meaning that the line did not pass straight through the graph. The derivative of with respect to at is, geometrically, the slope of the tangent line to the graph of at . The slope of the tangent line is very close to the slope of the line through and a nearby point on the graph, for example . These lines are called secant lines. A value of close to zero gives a good approximation to the slope of the tangent line, and smaller values (in absolute value) of will, in general, give better approximations. The slope of the secant line is the difference between the values of these points divided by the difference between the values, that is,