Throughout Welsh history the separate Welsh kingdoms came together only rarely. Most of the time they were too busy fighting each other.
In the second half of the ninth century Rhodri Mawr (Rhodri the Great) united large portions of Wales for several years. When he died his lands were divided, as was Welsh custom, between his six sons. They were unable to survive alone, so, along with rulers of other small territories, they gave homage to the English kings.
Two generations later in 900, his grandson Hywel Dda (Hywel the Good) became king of Seisyllwg, the modern counties of Ceredigion and Carmarthenshire. His marriage brought him Dyfed, and he called his territory Deheubarth. In 942 he siezed the kingdom of Gwynedd and later, Powys, unifying Wales. This only lasted until 949 when Hywel died, but in that time he did something very important.
Realising that the different territories in his kingdom each had their own laws and customs, Hywel Dda set about formulating a single set of laws which took the best of them and improved others. It is for these laws, known as Cyfraith Hywel Dda, that he is remembered. 
That time has many traditions and myths, making the truth difficult to establish, but at some time in the late 940s Hywel called a council which met for six weeks and decided on the laws, which were then written down. Many of the laws were very enlightened, giving women property rights and illegitimate children equal rights with legitimate ones.
Although Wales broke into small warring kingdoms after his death, Hywel Dda’s laws were still in use in Wales until the sixteenth century when Henry VIII joined Wales to England.
This is the fourth in my series on Welsh history, which arose from a report that children in Wales don’t learn enough of their own history.
So far, I have covered:
The Welsh, The Original Britons
The Power of the Druids
The Celtic Saints