Middle Welsh (Welsh: Cymraeg Canol) 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 /ɨ/ in North Welsh dialects and /i/ in South Welsh dialects, represented the close central rounded vowel/ʉ/ 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 marchawc = Modern Welsh marchog "horseman"). Similarly, the Middle Welsh diphthongs ei and eu have become ai and au in final syllables, e.g. Middle Welsh seith = modern saith "seven", Middle Welsh heul = modern haul "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 ei "his, her", eu "their" and the preposition i "to" are very commonly spelled y in Middle Welsh, and are thus spelled the same as the definite articley and the indirect relative particley. A phrase such as y gath is therefore ambiguous in Middle Welsh between the meaning "the cat" (spelled the same in Modern Welsh), the meaning "his cat" (modern ei gath), and the meaning "to a cat" (modern i gath). The voicedstop consonants/d ɡ/ are represented by the letters t c at the end of a word, e.g. diffryt "protection" (modern diffryd), redec "running" (modern rhedeg). The sound /k/ is very often spelled k before the vowels e i y (in Modern Welsh, it is always spelled c, e.g. Middle Welsh keivyn = modern ceifn "third cousin"). The sound /v/ 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 auall = modern afall "apple tree"). The sound /ð/ is usually spelled d (in Modern Welsh, it is spelled dd, e.g. Middle Welsh dyd = modern dydd "day"). The sound /r̥/ is spelled r and is thus not distinguished from /r/ (in Modern Welsh, they are distinguished as rh and r respectively, e.g. Middle Welsh redec "running" vs. modern rhedeg).