Sub-Roman Britain (Cavalry etc)

[cagwinn]

Where is the "th" arriving post 600 attested please?

See the chronology of Brittonic sound changes laid out by Kenneth Jackson in his "Language and History in Early Britain" (there is a significant body of literature on these sound changes now - in recent years Patrick Sims-Williams, Graham Isaac, Peter Schrijver, et al., have both done a lot of excellent work on "updating" Jackson's hypotheses - but it still remains one of the most influential books in British Celtic linguistics).

The Neo-Brittonic phoneme|th| likely developed during the 6th century (arising from the spirantization of the Brittonic geminate -tt-, along with later spirantization of the clusters -gt-, -ct-, -rt-, -ntr-, -ntl-, -ltr-, etc. by the Old Welsh period), however it continued to be written as -t- by scribes well into the medieval period (so that a name element like Welsh Arth- "bear" was still being spelled as Art- in the 9th century, for example). One needs to be careful to distinguish between phonological and orthographical changes - the two do not often keep pace with one another, since spelling conventions tend to be very conservative.