Rambam writes in his Introduction to The Mishnah: Afterwards, he [Rabbi Yehudah HaNasi] divided the material in the Order of Women [Nashim]. And he started with Tractate Yevamot (Levirate Marriages); and the reason that required him to start with Yevamot and not to start with Tractate Ketubot (Marriage Contracts) – and the investigation of the intellect ...


All orders [with the exception of Zeraim] are sorted by the size of the tractates, and Yevamot (16 chapters) is bigger then Ketuvot (13 chapters).


The first Tosofos Yeshanim in Mesechtas Yevamos asks why Yevamos comes first. We don't start with tragedies so Kiddushin should be first. He gives two reasons for why Yevamos was put first (1) Because Seder Nashim follows Seder Moed. Seder Moed ends with Moed Kotton (according to their ordering) so we put and study the tragedies near each other. (2) Because ...


The OU website notes a good reason: While Masechet Yevamot focuses on the rules and regulations concerning levirate marriage, nevertheless, it involves an examination of forbidden sexual and marital relationships, as well as the severity of those prohibitions. Thus, the study of Masechet Yevamot becomes a basic source for all of Seder Nashim.

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