SOURCE: Germanic */q/
OE /q/ had two different realizations, or allophones, depending upon its phonetic environment.
1. Voiceless interdental fricative--initially or in the vicinity of voiceless phonemes. Example: þing "thing."
REFLEX: ME /q/
2. Voiced interdental fricative--when surrounded by voiced phonemes. Example: pæðas "paths."
REFLEX: ME /ð/
SPELLING: þ, ð