The construction of Offa's Dyke in the late eighth century AD tells us that by that time the English language had advanced westwards in mainland Britain at least to a line corresponding more or less to the modern boundary between England and Wales. However, it was not until the early twelfth century that English made incursions into Welsh territory significant enough to mark the beginnings of the long process of anglicisation. These incursions came in the wake of the Normans, who established strongholds through the north and south of Wales. English speakers arrived in numbers, not only in the eastern border or ‘marcher’ lordship areas, but also in substantial parts of south Wales – the Vale of Glamorgan, the Gower Peninsula and south Pembrokeshire. It seems that some of these English speakers came to south Wales by sea, across the Bristol Channel, inaugurating south-west-of-England influenced dialects in Pembrokeshire and Gower. The process of anglicisation has involved social pressure as well as the westward advance of English, and by the time the Tudors came to the English throne at the end of the fifteenth century, it had become socially and politically advantageous for the upper classes of Welsh society to be able to speak English. The education system in the late nineteenth century further advanced the cause of English in Wales, but it was the Industrial Revolution which saw huge increases both in the population of Wales, particularly in the south-east, and in the proportion of that population speaking English.