The fourth and last chapter of Masechet Rosh Hashanah focuses on the mitzvah of shofar and the special davening—malchiut, zichronot and shofarot—for this special day. These two mitzvoth were joined together with the blowing of shofar taking place during the recital of the amidah. In fact, as far as the Mishnah is concerned, the shofar was blown only during the amidah.
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"Kol Yisrael areivim zeh bazeh, all Jews are co-signers [responsible] for one another" (Shevuot 39a). This is not just a nice idea reflecting the bond we must feel for all Jews everywhere; it is a legal principle, allowing one to perform certain mitzvoth on behalf of another. It is through mitzvoth that our community is strongest.