 If we think of a function \(y = f(x)\) as defining a curve, then, for any \(a\), the image under \(f\) of the “microinterval” \(\Delta + a\) obtained by translating \(\Delta\) to \(a\) is straight and coincides with the tangent to the curve at \(x = a\). In this sense each curve is “infinitesimally straight”.