It is widely observed that joint spacing is proportional to bed thickness in sedimentary rocks. The origin of this proportionality is explored by observation of joint spacing in the Monterey Formation of California and by one-dimensional numerical modelling based on Hobbs' theory of joint spacing. Cohesive rocks of the Monterey Formation—including dolostone, porcelanite, siliceous shale and chert show a nearly constant ratio of layer thickness to joint spacing of about 1.3. The frequency distribution of the ratio of joint spacing to median spacing is log-normal. Relatively pliable mudstones do not have regular joint sets but are mechanically important because they form the boundaries to the jointed, cohesive strata. Hobbs' model intuitively predicts a constant ratio of bed thickness to joint spacing; however, a simulation based on this model predicts a multimodal distribution of joint spacing. By adding the effect of a limited number of flaws to the model, which weaken the bed at random sites along its length, a simulated distribution of joint spacing is obtained that is similar to the observed log-normal distribution. Thus, Hobbs' model, modified to include the effect of flaws, seems capable of predicting the observed statistics of joint spacing as a function of layer thickness in sedimentary strata.