Annual estimates of productivity are reported for periods over 500 years for eight countries and for five other countries over shorter periods. One- and two-break time series models are used to investigate discontinuities in productivity growth. The results support two-break models of long-run productivity and they favour approaches to unified growth modelling with three epochs. However, the lessening of productivity gaps and the decisive shifts to higher productivity occurred in the twentieth century, chiefly in the years around the World War II. The timing of the breaks and the complexity of the historical record highlights a need for unified models to connect more closely with economic history.