Most Welsh surnames are of the 'patronymic' type, i.e. derived from the baptismal forename of the father or earlier direct ancestor. Originally, it was derived only from the father, i.e. that baptismal name of the father was the surname of the son. Welshmen preceded the name with "ap" (son of), if it started with a consonant, or "ab", if it started with a vowel. A surname was therefore transient, changing from generation to generation, which is why a Welshman was identified by his pedigree, for example:

	his name,	his father's name,	his grandfather's name,	his great-grandfather's name,	etc.
	Eynon	ap Iestyn	ap Rhys	ab Owen	etc

Surnames became permanent and were passed down unchanged in Wales much later than in England. The process started in the areas settled by the English, mainly in the Marches, and the southern parts of Monmouthshire, Glamorgan and Pembrokeshire, initially with the landed classes. It later worked its way down through the social order and across into the predominantly Welsh-speaking areas of Wales. It wasn't until the middle of the 19th century that this process became complete, as illustrated below the following table.

Patronymic surnames were formed by 'anglicizing' Welsh forenames with their prefixes in some cases, and without prefixes in others:

The above extract is from the certificate for a North Wales Registry Office marriage in 1840 (from the webmaster's own researches). The groom William ROBERTS is recorded as the son of ROBERT Griffith, and the bride Elizabeth JONES as the daughter of JOHN Thomas.

It proves surnames were still in a transitional, non-hereditary, state in Caernarvonshire in 1840.
There are even a few examples of surnames still in transition in the 1851 and 1861 censuses.

During the period of transition to hereditary surnames, brothers within the same family were sometimes known by slightly different 'anglicised' surnames. That is why family history research in Wales can be very challenging, to put it mildly. For example:
If HUW ap Tomos had three sons, Hywel, Rhys, and Evan,
they would originally have been known as Hywel ap HUW, Rees ap HUW and Evan ap HUW;
If the first son HYWEL ap Huw had two sons, Lewis and Henry,
they might have become known as Lewis POWELL, and Henry HOWELLS;
If the second son RHYS ap Huw had two sons, David and Joseph,
they might have become known as David PREECE, and Joseph REES;
If his third son EVAN ap Huw had two sons, Iestyn and Llewellyn,
they might have become known as Iestyn BEVAN and Llewellyn EVANS
So the 6 cousins, although all descended in the direct male line from their common grandfather Huw ap Tomos, might each have different surnames: POWELL, HOWELLS, PREECE, REES, BEVAN, and EVANS.