8. The seder plate should be arranged in the following manner: One should
place three matzos (1) on a plate and spread a beautiful cloth over them;
above them, one should place the "zeroa" (shankbone) on one's right side,
and the egg on one's left side; the "marror" (bitter herbs) over which the
blessing [for the mitzvah of marror] is recited, [should be placed below
them], in the center; the "charoses" should be placed below the "zeroa" (in
the next row down), and the "karpas" below the egg; the "marror" to use for
the "kricha" (sandwich with matzah) goes [below them] in the middle (2).
1) On every Shabbos and Yom Tov, it is a mitzvah to begin the meal with
two loaves of either bread or matzah ("Lechem Mishna"). On the Seder night,
however, we add a third matzah to be broken in half during the Seder
("Yachatz"). The reason we recite the Haggadah over a broken piece of
matzah, is because the Torah calls matzah "lechem oni" ("bread of
oppression" or "poor man's bread") (Deut 16:3); since a poor man is
accustomed to sustaining himself only on a morsel, so too, we use only a
morsel, as a reminder of our poverty and oppression in Egypt.
(2) This is the order of the Arizal, and is based on Kabbalistic teachings
(see Ba'er Hatev 473:8). The Ramah 273:4 has a different order based on the
principle of not passing over one mitzvah in order to get to another
mitzvah. As a result, according to the Ramah, those items used first should
be closest to you: bottom row - karpas (right), salt water (left) (the
Arizal doesn't have salt water on the plate); then the matzos in the next
row, in the center; third row - marror (R), charoses (L); top row - zeroa
(R), egg (L).
It seems that most people follow the order of the Arizal (See Oruch
HaShulchan 473:11, and Mishna Berura 473: 26).