Kitniyot

Although the Talmud specifically says that rice, millet, and other grains are kosher for Passover, during the Middle Ages, Ashkenazi rabbis worried that since flour ground from these and other grains,” legumes, and seeds, or kitniyot, could resemble one of the five hametz grains from which matzoh may be prepared, people might mistakenly eat food prepared with hametz, thinking it had been made with kitniyot. And since kitniyot looks like hametz, there may be grains of hametz mixed in with kitniyot (for example, bits of barley mixed in with grains of rice). To avoid such mix-ups, these rabbis banned consumption of all kitniyot during Passover.