There is an interesting Halacha in the name of the Vilna Gaon. Unlike most authorities, he ruled that we are to use two Matzot and not three, at the Seder.
This is based on decisions rendered by various Rishonim.
Another slight difference in the name of the Gaon, is his Psak regarding eating Matzot. Most opinions say that the real obligation to eat Matza, is at the Seder. The Torah clearly says, בערב תאכלו מצות, referring to the Seder.
He learns the Pasuk, שבעת ימים תאכל מצות, to teach that it may not be the same level of obligation as eating Matza at the Seder, but it is still a Mitzva to eat Matza all seven days.