Tohorot (Hebrew: טָהֳרוֹת, lit. 'Purities') is the sixth and last order of the Mishnah (also of the Tosefta and Talmud). This order deals with the clean/unclean distinction and family purity. This is the longest of the orders in the Mishnah. There are 12 tractates:[1]
According to Maimonides, the traditional reasoning for the order of the tractates is as follows:
The Jewish Encyclopedia, on the other hand, observed that the tractates are arranged in order of decreasing length.
There is a Babylonian Gemara only on Niddah. This is because most of the other laws of purity do not apply when the Temple in Jerusalem is not in existence. The Jerusalem Talmud only covers four chapters of Niddah.