Reprinted with permission from Maimonides’ Introduction to His Commentary on the Mishnah, translated and annotated by Fred Rosner, and published by Jason Aronson.
[After Seder Mo’ed (Appointed Times), Judah the Prince] then proceeded to subdivide the matter of Seder Nashim (Women) and commenced with tractate Yevamot (the levirate bride). The reason that compelled him to begin with Yevamot and not with tractate Ketubot (marriage contracts), since common sense dictates that the latter should more properly precede the former, is because marriage is related to a man’s wishes; and the courts do not coerce a man to marry a woman. However, regarding the Levirate marriage, they can do this by telling him: “Either perform Halitzah (renunciation of the Levirate obligation) or contract Levirate marriage.” It is more appropriate to begin with laws which are compulsory, and he therefore commenced with Yevamot and followed it with Ketubot. 
After Ketubot he lists tractate Nedarim (vows), because the entire scriptural portion dealing with vows speaks of vows of women as it is written: “Between a man and his wife, between a father and his daughter” (Numbers 30:17). When the marriage is completed by the woman coming under the canopy, the husband has the right to void her vows, and for this reason tractate Nedarim is next after Ketubot. After Nedarim he placed tractate Nazir (the Nazirite), because Nazirite oaths are also included among the laws of vows and if a woman should make a Nazirite vow the husband can void it and, therefore, he placed Nazir after Nedarim.
Having completed the discussion of matters related to marriage and the laws regarding the voiding of vows, he commenced the topic of divorce because after marriages come divorces; and thus he arranged tractate Gittin (divorce) after Nazir. And after Gittin is tractate Sotah (the suspected adultress) because its subject matter is related to the topic of divorce since if a suspected adulteress is found to have committed adultery one forces both the husband and the wife to go through with divorce proceedings, as I will explain in its proper place.
After Sotah he placed tractate Kiddushin (betrothals) and with it he completed Seder Nashim. One could ask at this point: Why is Kiddushin placed last? It would seem appropriate that it be earlier and listed before Ketubot. You might wish to answer and say that it was not listed before Ketubot in order not to separate Yevamot from Ketubot, since both deal with the same subject, namely, the matter of marriages of women so that their contents be tied together.
If so, Kiddushin should have been listed before Gittin in order to follow the logical sequence—first marriage and then divorce. The answer is that the sequence was so arranged because he wished to follow the sequence of the scriptural passage which speaks of divorce before marriage. This is what the Holy One, Blessed be He, stated: “And he shall write her a bill of divorcement and give it in her hand and send her out of his house, And when she is departed out of his house, she may go and become another man’s wife” (Deuteronomy 24:1-2). From the statement “she may become another man’s wife” we learn a fundamental teaching of the laws of marriage as is explained in the Talmud. “The process of becoming is compared to the process of departure.” And thus the subject matters of Seder Nashim are subdivided into seven tractates.
