Why is Yevamot the first masechet in Nashim?

Question

Rashi in Nazir (2a) says that Ketuvot is the main Masechta in Nashim.

If so, why is Ketuvot not the first Masechta in Nashim?

Thanks so much in advance.

Answer 1

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 would adduce that it is fitting to have it precede – is rather because marriage is something that stands in the discretion of a man and his will, and [so] the court is not able to coerce a man until he marries a woman; but with levirate marriage, it is compelled to do it and to say to him, "Either release or [marry]." And starting with things that are compelled before things that are not compelled is correct and fitting. And therefore, he started with Yevamot. (Source)

In other words, something one is compelled to do, such as in the case of Yevamot, is more of a priority than something that is based on one's own decision.

Answer 2

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 Yevamos discusses which women are permitted and which are forbidden to marry. That has to be learnt before we start with learning the laws of marriage.

Answer 3

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).

Answer 4

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.