A symmetric matrix of non-negative values for multidimensional Brownian motion and similar continuous valued models. Models of change at a molecular site. A matrix of non-negative instantaneous transition rates. 4 The general time reversible model and its varients. Models of change at a molecular site or a discrete organism character. This is Kimura's (1980) two parameter model. A model is something that estimates likelihoods of trees or character histories.