For now, let's concentrate on linear functions of two variables in the form

*z* = *mx* + *ny* + *b*.

(sections of a plane in x and y: z = -(1/3)x + 2y + 1)

What about the contour diagram for a plane? Setting *z* = *c* gives us the
graph of . These are
straight lines in the *xy*-plane with slope -*m*/*n* and *y*-intercept of
(*c*-*b*)/*n*.

Putting all of this together, we have a function whose sections in *x* are
parallel lines of slope *n*, whose sections in *y* are parallel lines of a
different slope, *m*, and whose contours are parallel lines with a slope
-*m*/*n*. What kind of surface will this form?