The ancient Welsh patronymic naming system can cause significant problems for genealogists. Patronymics describes the process of giving a male child the fathers given, or forename, as a surname. This means that a family's name changes in successive generations.

The process of conversion to the system of fixed names in Wales began in the fifteenth century and continued through to the middle of the eighteenth century. The trend was stratified socially - the higher classes in society began the process, which then was passed on to the lower classes. Consequently, genealogists whose search has reached this period in Welsh history can sometimes find that their search grinds to a halt as family names disappear into the patronymic system of naming.

The Welsh patronymic system describes family trees in terms of the male line only and records the family association in the 'ap' or 'ab' prefix ('ap' is a contraction of the Welsh word 'mab', which means son).
So Rhys ap Dafydd translates as 'Rhys, son of David'. Modern welsh surnames such as Powell, Price and Prichard are the result of this contraction and a progressive tendency to Anglicise Welsh names: under the patronymic system they would have been ap Hywel; ap Rhys and ap Richard. The names Bowen and Bevan were derived in the same way. Womens names sometimes entered the patronymic system using the ferch (daughter of) prefix. Rhiannon ferch Dafydd ap Iorwerth would be Rhiannon, daughter of David, son of Edward. When they married, women usually kept their maiden names as there was no surname for them to adopt.

The range of Welsh surnames is very small, due in part to this drawn-out process of conversion, but also because of the growing tendency to adopt English forenames (usually taken from Christian saints), particularly in towns like Hereford on the Welsh borders. Names such as John, William, David, Thomas and Hugh, became Jones, Williams, Davis, Thomas and Hughes. In north Wales, place names were frequently adopted, and in mid Wales families adopted nicknames for surnames. Jenkins is possibly derived from two different sources: as a corruption of a Flemish version of John, and as a result of the popularity of the name Ieuan in Wales during this period. Ieuan also gave rise to Evan(s) and Jones.

The way in which official records of births, marriages and deaths were kept also complicates the issue. The Act of Union (1536) stated that all official documentation in Wales was to be carried out in the English language. This meant that Welsh names were registered in an anglicised form. The process of civil registration in 1837 completed the long transition to fixed surnames. The traffic was not all one-way, though. The names of Welshmen who migrated to England were often transposed into English, so Ddu became Dee, and Caradog became Craddock.