While on a brief visit to Fermilab I was asked to look at the strengths of and locations for skew quads in the main injector. In this note we ill
first set up some notation in which to discuss the coupling and show how the coupling is generated and how it propagates. We will then
estimate the amount of coupling produced by two kinds of random errors, roll angles of normal quadrupoles and vertical orbit displacements
within sextupoles. Based on these we can make estimates of the strengths of the skew quads required to locally decouple the machine at one
point. also based on the way the coupling propagates we can identify good locations for the skew quads. This note presents no new
theoretical results; it only applies well known results to the specific case of the main injector.