and continues to do so, for instance by accepting loan words from other languages such as Latin and English. The Welsh language itself has many characteristics unfamiliar to most English speakers that can make it difficult to understand its place-names. For example, it uses a number of initial consonant changes (called "mutations") in different grammatical circumstances. In relation to place-names, this means that, for example, a parish (llan) dedicated to one of the saints Mary (Mair) becomes Llanfair – the initial m of Mair changes to f (pronounced /v/) for grammatical reasons. Other changes can apply to internal vowels. There are also differences between Welsh and English spelling standards, which have affected how place-names are spelled in the two languages. For instance, a single f in Welsh is always pronounced /v/ (or is silent), while ff is pronounced /f/; thus, the Welsh word for river, afon, is pronounced with a v-sound and so often spelled "Avon" when it appears in English place-names and river names. Development of place-names in Wales Early inhabitants of Wales gave names first to noteworthy natural features, such as rivers, hills, mountains, harbours and shores. Before the Roman occupation in the first century, there does not seem to have been a coming together in organised settlements, and therefore little reason to give names to such places. The Roman towns which were established were generally fortified and were given the generic name of castra, which in Welsh became caer, originally with the meaning of "fortified enclosure". Many of these continued as towns after the Romans left, including Caernarfon, Carmarthen (Caerfyrddin), Caerleon, and Caerwent. Elsewhere, many villages and later towns took their names from natural features. For example, Abergele refers to the "mouth of the [river] Gele", Harlech means "fair rock", Rhuddlan "red bank", and Porthcawl "harbour with sea-kale". Aberystwyth means "mouth of the Ystwyth", a river a mile or so away from the town centre, and was apparently so named as a result of confusion by the English over the different castles in the area. Many others took their name from religious settlements and clasau (monasteries) established from the fifth century onwards. These generally use the prefix llan, roughly equivalent to "parish". Most of them are dedicated to their founders, who hailed from local dynasties and were venerated as patron saints. Examples include Llandudno ("Saint Tudno's"), Llanelli ("Saint Elli's"), and Llantwit ("Illtud's"). Following the Norman invasion of