  This document has been motivated by a course entitled {\em From
Raviart-Thomas to HDG}, prepared for {\em C\'adiz Num\'erica 2013 -- Course and
Encounter on Numerical Analysis} (C\'adiz, Spain -- June 2013). It is an
introduction to the techniques for local analysis of classical mixed methods
for diffusion problems and how they motivate the Hybridizable Discontinuous
Galerkin method.
