Menachos chapter 3, mishnah 7, the last in the chapter. We continue to discuss mitzvos whose parts depend on one another: when the whole is made up of many components, and as long as a person has not made all of the components, he has not acquired the whole at all. Our mishnah lists five such laws.
The five laws in the mishnah:
"Shivah kenei menorah me'akvin zeh es zeh" - the seven branches of the Menorah hold one another back. This is the Menorah in the Sanctuary of the Beis HaMikdash. The verse required a central shaft and three branches on each side, seven in all. A menorah that has only six branches is not a valid menorah.
"Shivah neiros me'akvin zeh es zeh" - the seven lamps hold one another back. The Menorah itself has no built in cups; rather, there are seven holders on the seven branches, and on each holder rests a lamp. All seven lamps must be lit properly, and if there are not seven here, there is no valid Menorah.
"Shtei parshiyos shebamezuzah me'akvos zo es zo, va'afilu kesav echad me'akvan" - the two portions in the mezuzah hold one another back, and even a single letter holds them back. The mezuzah affixed to the doorway contains two portions: "Shema" and "Vehayah im shamo'a." In both of them it says explicitly, "and you shall write them on the doorposts of your house and on your gates," and therefore if one of them is missing, he has nothing at all. Moreover, even a single missing letter invalidates it, for the verse says "uchesavtam," and the Sages expounded: kesav tam - complete and perfect writing.
"Arba parshiyos shebatefillin me'akvos zo es zo, va'afilu kesav echad me'akvan" - the four portions in the tefillin hold one another back, and even a single letter holds them back. This law is stated in the mishnah before the law of the mezuzah. Tefillin contain four portions: two of them are the same as those of the mezuzah - "Shema" and "Vehayah im shamo'a" - and in addition, "Kadesh li kol bechor" and "Vehayah ki yevi'acha." If they contained only three, he has nothing at all; and likewise, if a single letter is missing, all the tefillin are invalid.
"Arba tzitziyos me'akvos zo es zo" - the four tzitzis hold one another back. These are the four tassels on the four corners of the garment. A four cornered garment on which tzitzis were placed on only three corners leaves him with nothing at all, since the mitzvah is four tzitzis on four corners. Moreover, ironically, he has here less than nothing: one who wears such a garment is actively transgressing a prohibition, since he placed tzitzis on a four cornered garment and there is no tzitzis on all of its corners.
The dispute of the Tannaim regarding tzitzis:
This last law is the subject of a dispute among the Tannaim. Rabbi Yishmael says that each one of the four tzitzis, on each and every corner, is a separate mitzvah in its own right, and in his view three are better than none.
The halachah, however, does not follow Rabbi Yishmael; rather, all four tzitzis must be placed on the garment. Therefore, one who does not have four valid tzitzis should not wear such a garment at all, since the very act of wearing it that way is a transgression.
In summary: This mishnah lists five laws in which the parts hold one another back: the seven branches of the Menorah, its seven lamps, the two portions of the mezuzah, the four portions of the tefillin - and in the last two, even a single letter holds them back - and the four tzitzis on the four corners of the garment. In all of them, the whole comes into being only when all of its parts are joined together.