Reference for Arakhin 15:10
פחות מסלע הוא דליכא הא יתר על סלע איכא
[if she says.] 'I saw uncleanness for two days', her re-count commences after seventeen days;<span class="x" onmousemove="('comment',' Similarly, if she had seen blood for two days, then the flow of blood again after seventeen days of cleanness is there-commencement of her days of niddah. If these two days were days of zibah then the days of niddah would commence no later than nine days after the flow, Or, if the two days of the flow were in the period of niddah then that period of niddah was over in five days, the following period of zibah terminated after eleven days, or the new period of niddah would re-commence after only sixteen days. But it is also possible that the first of the two days was the last day of the zibah period and the second the first of the niddah period, in which case six more days would be necessary to terminate her niddah period, to be followed by eleven days for her zibah period, so that seventeen days must pass before she can definitely be said to have become a niddah again.');"><sup>5</sup></span>