The Talmud poses the question as to the law regarding the “bein hash’mashot” (twilight) of Tishah B’Av: do the rules of the fast begin already at that point, or only once it is completely dark (tzet hakochavim)? The Rambam (Hil. Ta’aniyiot 7:2) rules that bein hashm’ashot is part of the fast. The Magid Mishnah explains the Rambam’s ruling as a reflection of the fact that Tishah B’Av, alluded to by a scriptural verse (Zechariah 8:13) has the status of “Divrei Kabalah” which is treated stringently in unresolved cases (safek) just as biblical law is. [See also Resp. Chazon Nachum, Tosefet Shvi’it, 67.] (This position, based on Rosh HaShanah 18b, is the often cited opinion of Turei Even, Megilah 5b, s.v. Chizkiyah; see also Taz, O.C. 687:2, citing Rabbeinu Tam; Sdei Chemed, vol. 2, pp. 63 and 258; Ohr Sameach, Berakhot 3:9; Resp. Torat Yerucham 3:26 and 3:82; Resp. Yachel Yisrael 46; and the Veroyer Rav (author of Sha’arei Torah) in the journal Kerem Shlomo, vol. 36, no 4, pp. 9-1l. However, many earlier authorities disagree; see Ran, Megilah and Ta’anit; Resp. Noda B’Yehudah, II, Y. D. 146, citing Rambam, and Rashi, Mikvaot 6:7, and Teshuvot HaMeyuchasot L’Ramban 263. See also Mishneh Berurah, 692:15, citing Pri Megadim, and R. Baruch Weiss, Birkhot Horai 23:11.) The Sefat Emet, however, explains the idea differently, drawing upon the concept of Tosefet Shabbat. Just as there is an obligation to begin Shabbat and Yom Tov early, Tishah B’Av has this concept as well. It should be noted that the Sefat Emet’s comments were made on the Talmud; the Magid Mishnah, in his words on the Rambam, understands him to be saying that there is no Tosefet for Tishah B’Av , because he requires starting at bein ha’shmashot, but no earlier, and that this is the view of the Ramban as well (see Mishnat Ya’avetz, O.C. 59). In Peirush HaMishnayot, however, the Rambam appears to recognize Tosefet for Tishah b’Av. The Resp. Shem MiShimon (O.C. 24) suggests that if the Rambam does recognize Tosefet for Tishah B’Av it is consistent with his view in general about the nature of Tosefet. In contrast to most other rishonim, who see Tosefet as a biblical obligation for every Shabbat and Yom Tov, the Rambam only mentions the concept (Hil. Shvitat haAsor 1:3) in the context of Yom Kippur, and in that case applies it not to the prohibition of melakhah but to the fast. If so, apparently he only recognizes Tosefet in relation to fasting; hence, he does so for Tishah B’Av as well. The question of whether Tosefet should be applied to Tishah B’Av would seem to hinge upen a question discussed by many authors as to the nature of Tishah B’Av and its stringency in relation to other fast days (In contrast to other rabinically ordained fasts, Tisha B’Av begins at night and prohibits all five “afflictions”, not only eating and drinking). This stringency may be a function of an obligation of mourning for the Beit HaMikdash. Alternatively, it may be a consequence of being modeled after the biblical fast of Yom Kippur. If the former possibility is true, Tosefet would apparently be inappropriate; mourning is never added on to, and is in fact shortened whenever possible. However, if Yom Kippur is the model, Tosefet would seem appropriate. Yom Kippur is certainly subject to Tosefet; as noted above, it is the one day that all rishonim agree has a biblical obligation of Tosefet (see Rosh HaShanah 9a), and is the model for all other days.