We argue for a model of document genre that encompasses both linguistic and graphical resources as a means of expression. Two bird books are analysed (1972 and 1996) and a simple framework presented for the description of graphical differentiation of communicative function. The analysis predicts that graphical differentiation in this genre is less in earlier publications. This suggestion is tested on a third bird book (1924) and found to be correct. However, the comparison shows that the genre itself has shifted over time, arguing for a 3-D model in which genre is represented as a shifting and interrelated set of parameters. Given this, analysis across genres and within genres across time is a very similar enterprise. A new project, GeM, has begun research into the appropriate model of genre for representing the generic differentiation of layout.