The shofar of Rosh Ha-shana is a hollow horn that grows from the head of an animal. The word “shofar” is related to the word “shefoferet” (“hollow tube”). A deer’s antlers may not be used, because they are not hollow but solid bone; one would have to drill a hole in them to use them. A cow’s horn may also not be used, as it is called “keren,” not “shofar” (Rosh Ha-shana 26a; SA 586:1). The sound comes out of the wider end of the shofar, the part that had been attached to the animal. This alludes to the idea that by blowing the shofar, we strike a blow against our animal nature, allowing us to reach a higher level of repentance.
There is a mitzva for the shofar of Rosh Ha-shana to be bent or curved, symbolizing the need for us to bend or humble our hearts before God (Rosh Ha-shana 26b). The best way to perform the mitzva is to use a curved ram’s horn. R. Abahu explains in the Gemara, “Why do we blow with a ram’s horn? God said, ‘Use a ram’s horn to blow before Me so that I will be reminded of the binding of Yitzḥak, son of Avraham, and I will consider it as if you sacrificed yourselves before Me’” (Rosh Ha-shana 16a). A mature male sheep is called a ram (ayil), while a female is called a ewe (raḥel). It is best to use the curved horn of the ram for a shofar; one step below that is to use the curved horn of the ewe; since ewes, too, are sheep, their horns also bring to mind the binding of Yitzḥak.
If a person has a choice of blowing either a straight ram’s horn or the curved horn of a different animal, it is preferable to use the curved shofar, because using a curved shofar is a law mentioned explicitly in the Mishna, while using a ram’s horn is only a preference (MB 586:5).
A ram’s horn is naturally curved, but if it grew straight, one may curve it by heating it up, thus making it into an ideal shofar. It is also permitted to heat a shofar and bend it for aesthetic reasons. (See Harḥavot.)
However, if someone heated it up and reshaped it so that the narrower end of the shofar became the wider end, he has invalidated the shofar. Similarly, if he turned a shofar inside out, it is invalid (SA 586:12). Such things can happen, because when a shofar is heated up it becomes very pliable; those who supply shofarot sometimes prefer to turn them inside out, because it is easier to smooth and beautify them that way. Therefore, a shofar must be bought from a trustworthy person, who can be believed when he says that he did not turn it inside out. When the wider end of a shofar has natural grooves and bumps, it is a sign that the shofar was not turned inside out.
We learn in a mishna, “All shofarot are kosher except that of a cow, because it is a horn” (Rosh Ha-shana 26a). Later on we learn, “The Rosh Ha-shana shofar should be the straight horn of an ibex…. R. Yehuda says, ‘On Rosh Ha-shana we blow rams’ horns’” (ibid. 26b). R. Yehuda is referring to curved rams’ horns, as horns from a ram are generally curved. R. Levi in the Gemara rules that on Rosh Ha-shana and Yom Kippur the mitzva is to use curved horns, because during those days the humbler a person is, the better. Thus, the first mishna teaches us that a cow’s horn is invalid because it is called a keren and not a shofar. In contrast, all other shofarot are kosher. The second mishna teaches us that it is preferable to use a curved shofar, but that a straight one is kosher as well. From R. Abahu’s statement in Rosh Ha-shana 16a we derive that the ideal shofar to use is a ram’s horn. This is the ruling of Raavad, Ramban, Rosh, Rashba, Ran, and many others. In contrast, Rambam rules that only a curved ram’s horn is kosher (MT, Laws of Shofar 1:1). According to him, R. Yehuda disagrees with the statement in the first mishna that all shofarot are kosher, believing that only a shofar from a bent ram’s horn is acceptable. This is also the opinion of R. Levi. R. Sa’adia Gaon and Yere’im follow this as well. However, the Aḥaronim rule in accordance with the majority of Rishonim that all shofarot are kosher. Therefore, one may recite a berakha before blowing a straight ibex horn or the like. This is also the ruling in SA 586:1. Nevertheless, in light of Rambam’s position, efforts should be made to procure a curved ram’s horn.