This article analyses the general relationships between general fertility indices and parity progression ratios by birth order. The former refer to the entire population whereas the latter concern persons who have had the same number of children. We establish a relationship between these two types of indicators by defining a mean parity progression ratio (PPR). This allows us to obtain a unified scale to represent these two types of indicators in a single graph. This mean probability can also be used to calculate fertility with small samples. Applications to different types of data, by cohort and by year, are presented. A decomposition of variations in general fertility indices as a function of variations in PPRs, including mean PPR, is also proposed. These relationships and methods are applied to the fertility of Spanish cohorts born between 1898 and 1970.