See Lauterbach, “Substitutes for the Tetragrammaton,” pp. 61–67. According to Lauterbach, new substitutes for writing the Tetragrammaton came into use in the early post-Talmudic period. Among these new abbreviations was three yods, which was in use as early as the eighth century C.E. (p. 59). This form, Lauterbach says, developed from YH, when the cursive H, which consists of two separated strokes, was written somewhat sloppily to look like two Y’s. The next stage was to drop one of the Y’s. Lauterbach’s theory can no longer hold water. We now know from the Sepphoris amulet that two Y’s were used as an abbreviation of the Tetragrammaton in the late fourth or early fifth century, right in the middle of the Talmudic period.