The spread of "sandhi h-" in thirteenth-century Welsh

After a hiatus following the Old Welsh period, Welsh manuscript evidence resumes c. 1250, and can now be studied in minute detail owing to the construction of a palaeographical chronology for the manuscripts and the availability of machine-readable and other modern editions. These reveal that the so-called ‘sandhi h-’ after first-person pronouns in modern literary Welsh is not ancient, but slowly emerged in the late thirteenth century as a hypercorrect phonetic tendency after nasal consonants which gradually became grammaticalised after pronouns ending in a nasal.