Models and comodels of algebraic theories, and the tensor product of a model with a comodel, were first studied by Freyd in 1966. This has subsequently become part of a rich strand of enquiry in categorical universal algebra (Bergman–Hausknecht, Tall–Wraith, Joyal, ...) – while its meaning for computer science was examined by Plotkin and Power in 2008. This talk, a warm-up for some original material to come, will explain the general idea, and the applications in computer science.