The Talmud does not mention the Seder plate, but it does say that “matza, lettuce, ĥaroset, and two cooked foods” are served to the person leading the Seder (Pesaĥim 114a). The Rishonim and SA (473:4) state that all of these foods should be placed on a plate. However, this is not obligatory. The main thing is that these foods be placed before the Seder leader. It is not necessary to place a Seder plate before each participant or even before each married participant. Rather, it is enough to place the plate before the Seder leader (MB 473:17). Nevertheless, some have a custom to place matzot before the head of every household, while the complete Seder plate is placed in front of the Seder leader only.
Since a number of foods must be placed on the plate, the question arises: what is the best way to arrange them? There are several opinions on this matter.
According to Rema, the principle is that the earlier a food appears in the Seder, the closer to the Seder leader it should be placed. This is done in order to avoid “passing over the mitzvot.” For example, if the matzot were closer to the leader than the karpas, he would have to pass over the matzot when reaching for the karpas, and this would be somewhat disrespectful to the matzot. Therefore, according to Rema, one should place the karpas and salt water closest to the leader, because these are eaten at the beginning, even before reciting the Hagada. Next come the matzot, which are eaten at the start of the meal. Then come the maror and the ĥaroset, because after eating matza we eat maror dipped in the ĥaroset. Furthest away on the plate are the zero’a and egg, which commemorate the Paschal and ĥagiga offerings.
Some say that there is no need to be particular about arranging the Seder plate in a manner that will prevent “passing over mitzvot,” because such behavior is only improper when one is presented with the simultaneous opportunity to perform two mitzvot. However, on the Seder night, each mitzva has a specific time of its own, and there is no problem in passing over a mitzva whose time for fulfillment has not yet arrived, in order to get to a food that must be eaten now.
The Seder plate arrangement based on Arizal’s teaching alludes to the ten kabbalistic sefirot. This arrangement is practiced today by most Sephardic, Ĥasidic, and even some non-Ĥasidic Ashkenazim. Other Ashkenazim follow Rema, while still others follow the Vilna Gaon. Many Hagadot contain diagrams of the Seder plate arrangement, and each of these varying customs has a place in Jewish law.