  When using the standard McCormick inequalities twice to convexify trilinear
monomials, as is often the practice in modeling and software, there is a choice
of which variables to group first. For the important case in which the domain
is a nonnegative box, we calculate the volume of the resulting relaxation, as a
function of the bounds defining the box. In this manner, we precisely quantify
the strength of the different possible relaxations defined by all three
groupings, in addition to the trilinear hull itself. As a by product, we
characterize the best double McCormick relaxation.
  We wish to emphasize that, in the context of spatial branch-and-bound for
factorable formulations, our results do not only apply to variables in the
input formulation. Our results apply to monomials that involve auxiliary
variables as well. So, our results apply to the product of any three (possibly
complicated) expressions in a formulation.
