Geoffrey Khan looks at imperfect performances of the prestigious Tiberian pronunciation tradition that are reflected in medieval Bible manuscripts. He proposes explanatory models for the development of such imperfect performances. Three factors are identified: interference of a less prestigious substrate, which he identifies as the Hebrew component of Jewish vernacular Arabic; hypercorrections; and varying degrees of acquisition of the Tiberian tradition. Khan describes these various phenomena and concludes that the imperfect performances must be datable to a period when the Tiberian pronunciation tradition was still alive and was familiar, though not perfectly, to the scribes.