Later, during the Koine Greek period, the aspirated and voiceless stops /tʰ d/ of Attic Greek lenited to voiceless and voiced fricatives, yielding /θ ð/ in Medieval and Modern Greek.
During what period did unaspirated and voiceless stops lenited to voiceless and voiced fricatives?
{'text': 'the Koine Greek period', 'answer_start': 14}