This book is about how to measure the relative stabilities of linguistic typological features. It discusses three alternative methods and tests their performance by applying them to simulated datasets having preset stabilities. The best metric is then applied to the data of The World Atlas of Language Stuctures (Haspelmath et al. 2005) to produce empirical estimates of stability for 134 features and 445 feature values. The numerical results concur with many specific categorical statements in the typological literature, and also substantiate the general suggestion that stable features are more strongly interrelated than are unstable features. The results also show that features have approximately the same relative rates of change in different languages, even in widely separated geographical areas. Surprisingly, however, tendencies for features to diffuse vary among areas with no consistent differences among features. It follows that stability and diffusibility are separate dimensions rather than opposite ends of the same dimension.