1. The problem statement, all variables and given/known data In double integrals, the change of variables is fairly easy to understand. With u = constant and v = constant, along line KL v = constant so dv = 0. Therefore the only contributing variable to ∂x and ∂y is ∂v. 3. The attempt at a solution However, in tripple integrals, you're simply adding one more w = constant that gives the 3rd dimension (the height to the initial flat-surfaced parallelogram before). So as you move along a line PQ (which is formed by say u = constant), then won't BOTH v and w contribute to ∂x, ∂y and ∂z since only u is constant?