welsh surnames: "The IGI takes 1 January 1813 as the cut-off date - before this date all IGI entries are listed using patronymic naming system, regardless of what the actual entry contained. In practice most people had already adopted surnames by 1812 and by the 1851 census examples are very few and far between.
The years of transition from patronymics were ones of confusion and it is essential to look for both patronymic and fixed surnames when researching families in the 18th century. In the IGI, searches should be done in the given name index as well as the surname index.
A man may have decided to use a fixed surname - but the village priest may have insisted on using patronymics in the parish register when he married him or baptised his children. Some people changed from patronymics to surnames half way through their families so that some children may use patronymics whilst their younger brothers and sisters use a surname. Sometimes a man would change from a patronymic to a surname at the time of his marriage - but his brothers may chose to continue with patronymics.
Every family is different and you need to be aware that surnames use was not standardised for a while"

Counties in Wales: " IGI Batch Numbers" Hugh Wallis

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welsh surnames