Fig. 3 in A Systematist's Guide to Estimating Bayesian Phylogenies From Morphological Data

Fig. 3. Schematic of an exponential (10) distribution. A commonly used distribution in Bayesian phylogenetics, the exponential is often used to place a prior on branch lengths. Under the exponential (10), most branch lengths are expected to be fairly short (to the left-hand side of the distribution), though longer branches are allowed.