Mesechet Menachot: The Taxonomy of the Gemara’s Grains
The Mishna of Menachot 70a lists the five species of grain. These species are of central importance in many halakhot. Only bread made from these species of grains is considered bread, and gets the brakha of hamotzi. Only matzah made from these grains is considered matzah, and can be used on the seder night. One only takes challah from bread made from these grains and no others. And it is only these grains which are forbidden prior to the bringing of the omer.
