The Torah writes "Keranos" two times without a 'Vov', and once with a Vov. The Mikra (the way words are pronounced) connotes six, and the Mesores (the way words are written) connotes four. Beis Hillel expound both, and consider it as if five were written.

Question: Beis Hillel should expound "Totafos (Tefilin)" similarly. Two are written without a 'Vov', and one with a Vov. They should require five compartments in the head Tefilin!

Answer: They hold like R. Akiva, who says that the word itself means four. 'Tat' means two in Katfi (a language). 'Fos' means two in Afriki.

Menachos 34b (Beraisa) Suggestion: Perhaps the four Parshiyos should be written on four parchments, and each is put in its own Bayis (box) made from a separate piece of hide!

Rejection: "Ul'Zikaron Bein Einecha" - it should be only one commemoration;

To fulfill both verses, the four Parshiyos are written on separate parchments, each is put in its own Bayis, and all four Batim are made from one piece of hide;

If the Parshiyos were written on one parchment and (the Klaf was folded like an accordion, so) each Parshah was in its own Bayis, this is Kosher.

Rebbi says, there must be a space between them;

Chachamim say, there need not be.

All agree that a string must separate the Batim (some say, only if the Parshiyos are on one parchment). If the separation between Batim is not recognizable (from the outside), it is Pasul.

32a (Rav Nachman bar Yitzchak): L'Chatchilah, the Parshiyos in a Mezuzah should be closed. (We leave different amounts of empty spaces before open and closed Parshiyos.) R. Shimon ben Elazar teaches that even open Parshiyos are Kosher.

Support (Beraisa): If (parchments of) a Sefer Torah or Tefilin became worn, they may not be used for a Mezuzah, for we do not demote something to a lower Kedushah.

Inference: If it were permitted to demote to a lower Kedushah, they could be used for a Mezuzah, even though the (first) Parshah (in a Mezuzah, Shma Yisrael) is closed!

Rejection: The Beraisa teaches that one may not take from a Sefer Torah to complete a Mezuzah.

33a (Rav Yehudah): If a Mezuzah was written in two Dapim, it is Pasul.

Question (Beraisa): If a Mezuzah was written in two Dapim and one was put on each doorpost, it is Pasul.

Inference: If both were put on the same doorpost, it would be Kosher!

Answer: It means that if it is possible to put one Parshah on each doorpost, it is Pasul.

Sanhedrin 89a: If one made four Batim for Tefilin and put a fifth next to them, it does not invalidate the Mitzvah. If he made five compartments from the beginning, it was invalid from the beginning!

Mishnah (Kelim 18:8): Tefilin is four Kelim. If (it was Tamei Mes, and) he untied the first Bayis and fixed it, it (the Tefilin) is Tamei Mes. Likewise, if he (later) untied the second and third Bayis and fixed them, it is Tamei Mes. If he (later) untied the fourth Bayis and fixed it, it is not Tamei Mes, but it is Tamei like something that touched a Tamei Mes.

Rif and Rosh (Hilchos Tefilin 6b and Siman 2): One puts the four parchments of the head Tefilin in four Batim in one hide. If he wrote them on one parchment and put them in four Batim in one hide, he was Yotzei.

Rambam (Hilchos Tefilin 2:1): One puts the four parchments of the head Tefilin in four Batim in one hide.

Tosfos (32a DH Dilma): Rashi explained that if not for the Isur to demote a Sefer Torah to a lower Kedushah, one could take a line from a Sefer Torah to complete a Mezuzah missing that line. The Yerushalmi says that Tefilin and Mezuzos may be written only on one parchment. It disqualifies two parchments only when they are not sewn together. Alternatively, our Gemara discusses taking a full Parshah (Shema or v'Hayah) from a Sefer Torah to complete a Mezuzah.

Rashi (33a DH Re'uyah): 'It is possible to put each Parshah on a different doorpost' means that there is space between the columns to cut the parchment.

Shulchan Aruch (OC 32:38): One makes four Batim from one piece of hide for the Shel Rosh, and one Bayis for the Shel Yad.

Prishah (34): My Rebbi (the Maharshal) explained that this is because there are four senses in the head (sight, hearing, smell and taste), and one in the hand (feeling), and one blesses his Creator for all of them.

Beis Yosef (DH v'Ya'aseh): Sefer ha'Terumah says that the four Batim are connected, from one hide. Or, perhaps they may be from four hides, as long as they are sewn together. It is proper to make them from one hide.

Taz (32): Regarding the Shel Yad it says "Lecha l'Os" - just like they are one sign on the outside, they are one sign inside.

Bach (21): The Gemara connotes that they must be from one piece of hide, without sewing, unlike Sefer ha'Terumah. This is the custom throughout Yisrael.

Magen Avraham (52): The Agudah proved that they are like one hide (when they are sewn together). Also Tosfos (32a) says so (about the parchments).

Gra: Rashi, Shimusha Rabah and the other Poskim say that it must be from one hide. Even Tosfos said that when they are sewn they are like one parchment only according to one answer.

Chasam Sofer (OC 5): Rashi (33a) explains that Rav Yehudah disqualified a Mezuzah in two columns, even if they are on one parchment. R. Elchanan (in Mordechai Hilchos Mezuzah 961) supported this from the Yerushalmi. R. Yehoshua ben Levi suggested that a difference between Seforim and Mezuzos is that Seforim may be written in two Dapim, but Mezuzos may not. The Yerushalmi asked, why didn't he suggest a greater difference, that Seforim may be written on two parchments, but Mezuzos may not? This shows that 'two Dapim' are on one parchment, and even so it is Pasul due to two Seforim. Regarding Sotah, it says (that the curses must be written) "b'Sefer", i.e. the most clear-cut Sefer, so it is Pasul even if it is on one parchment in two columns. Regarding Get, it says ("Sefer", but it does not say) "b'Sefer", so if it is in two columns on one parchment it is Kosher, but in two parchments sewn together it is Pasul. A Tosefta says that if a Get was torn, it does not help to sew it back. Regarding a Sefer Torah it says "Al Sefer", and it is written on many parchments. They are sewn together, and they become one Sefer. Therefore, the Drasah 'one Sefer, and not two or three Seforim' disqualifies when the parchments are not tied together. It does not say 'Sefer' regarding Mezuzah, but it says "u'Chsavtem", and we are stringent to learn from Sotah that it is Pasul in two columns.

Chasam Sofer (ibid.): It is clear from the Mishnah in Kelim that the custom used to be to make Tefilin from four pieces that could be tied together and untied. Also Sanhedrin 89a connotes like this. However, the Beraisa concludes 'if the grooves are not recognizable, they are Pasul.' Rashi explains that one may not make one Bayis with walls separating in between. Rather, the grooves must be recognizable from the outside. Why didn't Rashi say that the four Batim must be separated? Rashi hints that it is the custom of scribes to stick four Batim together with glue, lest they separate and the Tefilin will not be square. Even if one is skilled to make them square without glue, it would not remain square without glue. Sometimes, through glue it is not clear that they are four Batim and it is Pasul. This shows that sticking separate things together does not make them like one. If not, how does a recognizable groove help, if the four Batim are like one? Rather, they do not become like one. This is unlike the Mishnah in Kelim! We can say that sewing is better than gluing. Even though regarding Shabbos, gluing is a Toldah of sewing, to make one rememberance sewing is better. The Rosh (Hilchos Sefer Torah 14) says that one can sew a torn Sefer Torah with sinews, but he cannot glue it. In truth, some Poskim connotes that gluing is better than sewing. This is when the issue is not two Seforim, e.g. it tore within three. Then, gluing is better. For a connection, sewing is better.

Magen Giborim (32:28) asked from Shabbos 28b, which says that only hide of Kosher animals may be used for Melachah of Shamayim. The Gemara asked that we already know that a Kosher hide is needed (for Tefilin) due to the Shin on the Tefilin (a letter of Hash-m's name), which is a tradition from Sinai! If sewing helps, perhaps the Tefilin could be from a Tamei animal, and the Shin would be from a Tahor animal and sewn on! I say that surely this does not help, just like a woolen Talis with leather corners requires Tzitzis on the leather corners, for sewing makes it one. A leather Talis with woolen corners is exempt, for it is primarily leather. Also here, we follow what is primary. A Shin on Tahor hide on a Tamei Bayis is Pasul, as if it were written on Tamei hide.

Chasam Sofer (ibid.): Sewing does not help for a Get. Why does it help for Batim? Maharam Mintz says that the Beraisa does not require a Bayis from one hide. Rather, even if he made it from one hide, it is Kosher. One might have thought that four hides are needed, like we expounded "Totafos." This is unlike the Gemara, which says 'one rememberance, not two or three.' It did not say 'one suffices.' Also, it should have taught this together with 'if he wrote them on four parchments, or on one parchment and put it in four Batim in one hide, it is Kosher. Rather, the Reisha teaches that one must put them it in four Batim from one hide, and the Seifa teaches that one was Yotzei b'Di'eved (if they were on one parchment). It says regarding Tefilin "Zikaron" and Kesivah, so we learn from "Kesov Zos Zikaron b'Sefer", which refers to a Sefer Torah, which is written on parchments sewn, but not glued, together. Likewise, the Batim may be sewn, but not glued, and this does not make them one Bayis.

Mishnah Berurah (172): The Chayei Adam and Or Zaru'a connote that even if they are glued together, it is Kosher. Nowadays, this is the custom in our lands. L'Chatchilah, it is proper to make them from one hide, for some hold that sewing or gluing does not help. Chayei Adam says that in any case the Batim must be separated. They cannot be stuck together, like scribes do. They glue them together, just above they make a groove for a mere Siman. It is not clear if this is Kosher b'Di'eved. If gluing makes them like one hide, there is only one Bayis. If gluing does not make them like one hide, the Tefilin were made from multiple hides, and are Pasul! Even those who make Tefilin from one hide should separate them, for perhaps gluing makes them like one. The Gra's Tefilin were separated.

Bi'ur Halachah (DH Ya'aseh): The Or Zaru'a and Machatzis ha'Shekel equate sewing and gluing. Ha'Gaon R. C. Tzanzer says that all agree that gluing help; it is better than sewing. The Chasam Sofer says that all agree that gluing does not help; it is worse than sewing. Machatzis ha'Shekel says that if it was from one hide, but it was cut into strips and left connected in the middle, and afterwards the strips are connected, according to the Bach it is Pasul (for he holds that sewing and gluing do not help). The Pri Megadim is Machshir b'Di'eved. I agree, for the stringent opinion compares Tefilin to a Get. Gluing does not help for a Get, but if a Get is barely connected, it is called one Sefer (Gitin 20b). In such a case with Tefilin, all agree that sewing or gluing helps.