This page summarises the classic Stern-Gerlach experiment on "spin" and extends the treatment to a discussion of correlation experiments. As is often the case, I build up maximum complexity as I examine the experimental details, and then hide them in a 'box'. This time the box will turn out to be literal. Here we concentrate on electrons, which have only two spin-states. We also mention photons, which also have two spin-states. The approach is largely based on one by Feynman which he used for objects with three spin states: see R.P. Feynman, R.B. Leighton and M. Sands, The Feynman Lectures on Physics, Vol III, Chapter 5 for this discussion.