Wikipedia
Middle Welsh is the label attached to the Welsh language of the 12th to 14th centuries, of which much more remains than for any earlier period. This form of Welsh developed from Old Welsh.
Middle Welsh is the language of nearly all surviving early manuscripts of the Mabinogion, although the tales themselves are certainly much older. It is also the language of most of the manuscripts of Welsh law. Middle Welsh is reasonably intelligible, albeit with some work, to a modern-day Welsh speaker.
The phonology of Middle Welsh is quite similar to that of modern Welsh, with only a few differences (Evans 1964). The letter u, which today represents in North Welsh dialects and in South Welsh dialects, represented the close central rounded vowel in Middle Welsh. The diphthong aw is found in unstressed final syllables in Middle Welsh, while in Modern Welsh it has become o (e.g. Middle Welsh = Modern Welsh "horseman"). Similarly, the Middle Welsh diphthongs ei and eu have become ai and au in final syllables, e.g. Middle Welsh = modern "seven", Middle Welsh = modern "sun".
The orthography of Middle Welsh was not standardized, and there is great variation between manuscripts in how certain sounds are spelled. Some generalizations of differences between Middle Welsh spelling and Modern Welsh spelling can be made (Evans 1964). For example, the possessive pronouns "his, her", "their" and the preposition "to" are very commonly spelled in Middle Welsh, and are thus spelled the same as the definite article and the indirect relative particle . A phrase such as is therefore ambiguous in Middle Welsh between the meaning "the cat" (spelled the same in Modern Welsh), the meaning "his cat" (modern ), and the meaning "to a cat" (modern ). The voiced stop consonants are represented by the letters t c at the end of a word, e.g. "protection" (modern ), "running" (modern ). The sound is very often spelled k before the vowels e i y (in Modern Welsh, it is always spelled c, e.g. Middle Welsh = modern "third cousin"). The sound is usually spelled u or v, except at the end of a word, where it is spelled f (in Modern Welsh, it is always spelled f, e.g. Middle Welsh = modern "apple tree"). The sound is usually spelled d (in Modern Welsh, it is spelled dd, e.g. Middle Welsh = modern "day"). The sound is spelled r and is thus not distinguished from (in Modern Welsh, they are distinguished as rh and r respectively, e.g. Middle Welsh "running" vs. modern ).