Summary Model fertility schedules based on the proportions married and the age pattern of marital fertility are unsatisfactory to the extent that marital fertility does not depend on age alone but also on other factors. Most notably, models based just on age fail to allow for differences between populations in their composition by marriage duration. Examination of the major series of fertility rates specific by both age and duration of marriage (Sweden 1911-70, England and Wales, 1941-70) reveals striking underlying regularities. The marital fertility rates observed at any given point in time can be factored into three independent components - an overall level, a vector of age effects common to all marriage durations and a vector of duration effects common to all age groups. A simple product of these three components is shown to approximate the data very closely over the entire series, despite major changes in the aggregate levels of fertility and nuptiality during the periods concerned. Not only are the data tightly structured, conforming very closely to this simple multiplicative model, but the vectors of age and duration effects themselves are shown to exhibit clear and meaningful regularities.