  Hidden variables are extra components added to try to banish counterintuitive
features of quantum mechanics. We start with a quantum-mechanical model and
describe various properties that can be asked of a hidden-variable model. We
present six such properties and a Venn diagram of how they are related. With
two existence theorems and three no-go theorems (EPR, Bell, and
Kochen-Specker), we show which properties of empirically equivalent
hidden-variable models are possible and which are not. Formally, our treatment
relies only on classical probability models, and physical phenomena are used
only to motivate which models to choose.
