Razor variables were introduced in 2010,1 as a means of dealing with QCD backgrounds in squark and gluino searches. The basic principle is this: if we have events with substantial amounts of missing energy (such as events with dark matter production - the dark matter escapes the detector entirely), then we cannot completely reconstruct what happened in the collision, that is, we cannot get to the center-of-mass frame - we just do not have enough information. But for a particular kind of process, we can reasonably approximate it. Suppose we have pair production of two nearly mass-degenerate particles $$S_1$$ and $$S_2$$, which decay respectively into $$Q_1\chi_1$$ and $$Q_2\chi_2$$, with $$\chi_{1,2}$$ being invisible. If we further assume that $$S_{1,2}$$ are produced at threshold, that is, all the energy from the incoming particles has been used to produce $$S_{1,2}$$, with no leftover energy to give them kinetic energy. Under these condidtions, we can perform a boost to a razor frame, which approximates the center-of-mass frame reasonably well.