Yevamos 2a: The Mishna Rishonah and Tzaras Tzarah's Need for a Pasuk (Part 1)

This shiur, the first in Maseches Yevamos, begins by establishing the standard formula for the cases that will recur throughout the masechta: Reuven (the original mes), Shimon (the yavam who later becomes the second mes), and Levi (the one to whom Reuven's wife is an ervah). The opening Mishna presents the din of tzaras tzarah: when Levi's daughter (bito) is married to Reuven, and Reuven dies, both the daughter and her co-wife (Leah) are patur from yibum. If Leah then marries Shimon and Shimon dies, Leah's co-wife (Bilhah) is also patur—this is called tzaras tzarah, the co-wife of a co-wife. Rabbi Zweig opens with a fundamental linguistic problem in the Mishna. The Mishna says "kach tzaras tzarasah peturah"—literally, the co-wife of her co-wife is patur. But this is peculiar. By the time Leah marries Shimon, Leah herself has become an ervah on Levi—not merely as bito's co-wife, but as eshes ach shelo b'makom mitzvah (מצוה). An eshes ach shelo b'makom mitzvah is a full-fledged ervah (like eshes ach shelo hayah b'olamo). Once Leah is an ervah, Bilhah (Shimon's wife) should be patur simply because she is tzaras ervah—the co-wife of an ervah. Why does the Mishna call her "tzaras tzarasah" (tracing her status all the way back to Rochel, Levi's daughter) rather than simply "tzarasah" (the co-wife of Leah, who is now an ervah)?