the beginning of menstrual cycles in are essential laws. Tractate Kinnim in the Mishnah interpreted the laws of pairs of sacrificial pigeons and doves in , , , , and ; and . Interpreting the beginning of menstrual cycles, as in , the Mishnah ruled that if a woman loses track of her menstrual cycle, there is no return to the beginning of the niddah count in fewer than seven, nor more than seventeen days. The Mishnah (following ) taught that a sin-offering of a bird preceded a burnt-offering of a bird; and the priest also dedicated them in that order. Rabbi Eliezer taught that wherever an offerer (because of poverty) substituted for an animal sin-offering the offering of two birds (one of which was for a sin-offering and the other for a burnt-offering), the priest sacrificed the bird sin-offering before the bird burnt-offering (as instructs). But in the case of a woman after childbirth discussed in (where a poor new mother could substitute for an animal burnt-offering two birds, one for a sin-offering and the other for a burnt-offering), the bird burnt-offering took precedence over the bird sin-offering. Wherever the offering came on account of sin, the sin-offering took precedence. But here (in the case of a woman after childbirth, where the sin-offering was not on account of sin) the burnt-offering took precedence. And wherever both birds came instead of one animal sin-offering, the sin-offering took precedence. But here (in the case of a woman after childbirth) they did not both come on account of a sin-offering (for in poverty she substituted a bird burnt-offering for an animal burnt-offering, as required her to bring a bird sin-offering in any case), the burnt-offering took precedence. (The Gemara asked whether this contradicted the Mishnah, which taught that a bird sin-offering took precedence over an animal burnt-offering, whereas here she brought the animal burnt-offering before the bird sin-offering.) Rava taught that merely accorded the bird burnt-offering precedence in the mentioning. (Thus, some read Rava to teach that lets the reader read first about the burnt-offering, but in fact the priest sacrificed the sin-offering first. Others read Rava to teach that one first dedicated the animal or bird for the burnt-offering and then dedicated the bird for the sin-offering, but in fact the priest sacrificed the sin-offering first.) called for "two turtle-doves, or two young pigeons: the one for a burnt-offering, and the