Until the Tudor period Welsh names generally took the format X son of Y, or X daughter of Y. The Old Welsh word for son was "map", but it lenited to "fap" (today "fab"), then "". Although some of the Welsh gentry began to adopt the English fashion for hereditary surnames even before the Act of Union between England and Wales in 1536, in general the Act marked the beginning of a gradual shift towards hereditary surnames in Wales that was not complete until the 18th century. In some cases "ap" was merged into the following personal name to create an hereditary name. So "Ap Richard" became Pritchard, "ap Even" became Bevan and "ap Hugh" emerged as Pugh. The shift from "ap Rosser" to Prosser is of this pattern, though the surname Rosser also occurs. The distribution of Prosser and Rosser in 1842-46 was strongly centred in south-east Wales, and largely absent from other parts of Wales.