The seven branches of the menorah are essential to [the validity of] one another. Its seven lamps are essential to [the validity of] one another. The two [Scriptural] passages which are in a mezuzah are essential to [the validity of] each other, and even the writing of one [letter] is essential to them. The four passages which are in tefillin are essential to [the validity of] one another, and even the writing of one [letter] is essential to them. The four fringes of the tzitzis are essential to the validity of one another, as they are all four one mitzvah. R’ Yishmael says: The four of them are four mitzvos.