The Talmud teaches that one who mistakenly misses the time for prayer has the opportunity to make up that prayer during the next time slot (tashlumin). The definition of “mistakenly” is the subject of some discussion. Many authorities allow for a very broad definition; the Arukh HaShulchan (O.C. 108:2) includes anything other than “absolutely intentional” (mezid gamur) {See Rambam, Hil. Tefilah 3:8, Shulchan Arukh, 108:8, with Rama and Mishnah Berurah, and Resp. Teshurat Shai, II, 17). Further, the Shulchan Arukh (108:7) allows one who intentionally skipped prayer to offer a “voluntary” prayer (n’davah). The Taz explains that the apparent desire to repent converts the intentional to the unintentional.

This possibility of tashlumin prompts a question as to the nature of the institution of three daily prayers. One perspective may indicate that prayer is a general obligation, with three time slots every day. Another may suggest that every prayer constitutes an individual obligation, with tashlumin signifying the possibility of making up one prayer during the time of another.

The rules of tashlumin dictate that the prayer that is currently required is recited before the make-up prayer, a detail that is disqualifying if disregarded (The Magen Avraham and the Arukh HaShulchan debate whether or not some outward indication of disregarding this rule is necessary). The Arukh HaShulchan wonders why this is; he notes that some (such as the Levush) explains this as the preference for the more common obligation (tadir kodem) but observes that that issue is generally not disqualifying. He also cites an opinion that the rule is a penalty to prevent intentional deferral of prayer, but he rejects this as well. He concludes that the reason is that for one who has to prayers to recite, one currently mandatory and another a make-up, to recite the make-up first would be a willful neglect of the mandatory prayer and an offense to G-d. The Kogaglover Rav (Resp. Eretz Tzvi, I, 87) explains differently: The Rabbis have ordained that the tashlumin must take place during a time of prayer; thus, the timely prayer must be recited first, to create a “time of prayer” (See also Resp. HaElef Likha Shlomo, O.C. 53). These two explanations reflect the above possibilities. The Arukh HaShulchan is perceiving two separate obligations of prayer; the Kogaglover Rav is describing one general obligation that is fulfilled in distinct “time slots”.

The above question is relevant as well to the issue of what exactly is accomplished by tashlumin. Is it to be understood that the make-up prayer actually replaces the missed prayer, or, is it rather, that filling in a prayer after one has been missed is valuable in that it preserves the three-prayer-a-day structure, without necessarily adopting qualities of the missed prayer.

The Tur (O.C. 108) cites the Behag that it is possible for one who missed both ma’ariv and the following shacharit to make up both with three prayers in the time for minchah. Such an opinion seems to indicate that the purpose of tashlumin is to maintain the three-a-day structure. The practice, though, is to only allow tashlumin for one prayer (The Pri Megadim suggests this is because going past more than one "time zone" suggests intentional neglect of prayer; see Resp. Shraga HaMeir, VIII, 132.)

Most rishonim (other than the Meiri) agree that musaf is not part of the tashlumin cycle; they provide at least three possibilities why this would be, all addressing unique properties of musaf. This may suggest that the goal of tashlumin is to replace the previous prayer, and that musaf is not conducive to that. Alternatively, working off of the other perspective, it may be that musaf is excluded because it is not part of the three-a-day cycle. (As to n'eilah, see Resp. Shevet HaLevi II, 67; IV, 80; and V, 67.)

Tosafot (26b s.v. ta’ah) notes that one who needs to make up a missed minchah from Shabbat afternoon on motza’ei Shabbat shouldn’t worry about the disparity between the short Shabbat amidah and the longer weekday version, since in theory Shabbat would have that version as well, were it not for fact that the Rabbis removed personal requests in honor of Shabbat. The Ritva makes a similar comment concerning the inverse situation. These comments indicate that in theory the prayers would have to match, perhaps implying that tashlumin serves to make up the missed prayer. (See also Resp. Yabbia Omer, V, 14, in reference to the internal issue of the various prayers on Shabbat itself).

Rishonim argue, however, on the question of one who omits Ya’aleh v’yavo in the amidah of minchah on Rosh Chodesh – which requires the amidah to be repeated – and only realizes after Rosh Chodesh is over. Some rishonim (the Rif, Chakhmei Provence quoted in Tosafot) maintain that at Ma’ariv, the amidah should be said twice. Others (Tosafot, ibid, Rosh, Ritva, Rashba, Meiri) felt that it was pointless to add an amidah at that point, as Rosh Chodesh is already over and Ya’aleh v’yavo will not be recited in any event. This dispute seems to touch upon two points: 1) the above question about the role of tashlumin; (see Sh’erit Yosef, IV, 7); 2) perhaps more significantly, the question of the nature of the obligation to repeat the amidah if Ya’aleh v’Yavo is omitted: does that indicate that Ya’aleh v’yavo is so crucial, it justifies an extra amidah, as long as that will provide an opportunity for its inclusion; or does it indicate that an amidah on Rosh Chodesh without Ya’aleh v’yavo is completely invalid, so that it requires a make-up, even if that make-up will not een include Ya’aleh v’yavo.

This latter question affects a number of other issues, including: a) one who, on a combined Shabbat/Yom Tov, omits the reference to Shabbat, repeats the prayer, that time omitting the reference to Yom Tov: is a third prayer required? (See Resp. Hitor’rut Teshuvah, O.C., I, 122, and Ikvei Sofer). b) Similarly, one who omits “ten tal u’matar l’vrakhah” on Rosh Chodesh Kislev, and repeats, omitting Ya’aleh V’Yavo (see Resp. Har Tzvi, O.C. I, 54, arguing with Chiddushei HaGrach al haShas, who distinguishes between Ya’aleh V’Yavo and ten tal u’matar l’vrakhah; see also Resp. Teshuvot V’Hanhagot, I, 90); c) one who is already obligated in a tashlumin amidah, and first must precede the timely prayer, and omits Ya’aleh V’yavo; must it be attended to immediately, before tashlumin, or can it be repeated afterward? (See Pri Chadash, citing R. Shmuel Germaizin, and Resp. Eretz Tzvi, I, 40), d) whether a minyan should be sought after for the repeated amidah; e) in the case of shacharit, if the repition must take place with in the first four hours of the day, the time allotted for shacharit (see Resp. VaY’varekh David, 158); and f) if tefilin should be worn (see Eishel Avraham of Butzacher Rav, 25:12, and Resp. Shraga HaMeir, VII, 68:2).

The Rashash writes that one who is reciting the amidah twice at ma’ariv, the second is a “reshut” as that is the status accorded to ma’ariv. Based on this, the Resp. L’Horot Natan (V, 42) explains the opinion of the Maharil (135) that one who missed minchah on Friday can accomplish the goal of tashlumin by listening to the recitation of “Magen Avot” in synagogue, as that prayer contains the essential elements of the amidah, and the opinion of the geonim (quoted in Beit Yosef, 268) is that one could fulfil the obligation of ma’ariv in that manner. The Orchot Chaim (O.C. 108:9, citing Minchat Pitim) objects, noting that this position is based on the “reshut” status of ma’ariv; the Tashlumin, though, would be a stand-in for minchah. The Rashash’s view, though, suggests that the tashlumin prayer is not to replace the earlier prayer exactly, but to preserve the overall number of prayers. (Contrast the view of Resp. HaElef Likha Shlomo, O.C. 53, and see Resp. Yabbia Omer, VI, 19).