The Torah commandment to eat maror on the night of the fifteenth of Nisan is contingent upon the eating of the Paschal sacrifice, as it states: “They shall eat it with matzot and merorim [plural of maror]” (Bamidbar 9:11). Since we are unable to offer the Paschal sacrifice today, the mitzva to eat maror is now rabbinic (Pesaĥim 120a). We dip the maror in ĥaroset in order to counteract its bitterness, and then we shake off all the ĥaroset that sticks to it, because ĥaroset should not be eaten with the maror (SA 475:1, MB 13 and Kaf Ha-ĥayim 23 ad loc.). After this we recite the berakha “Who sanctified us with His mitzvot and commanded us concerning eating maror” (“asher kideshanu be-mitzvotav ve-tzivanu al akhilat maror”), and eat a kezayit of maror.
The Sages enumerate five types of maror: ĥazeret, tamĥa, ĥarĥavina, ulshin, and maror. Today we are only familiar with two of these: ĥazeret, which is lettuce, and tamĥa, which is horseradish. The Sages stated that the choicest type of maror is lettuce, and its Hebrew name (“ĥasa”) even alludes to the fact that God has mercy (“ĥas”) upon us. They also tell us that the Egyptian enslavement was like maror: just as maror begins soft and ends hard – the stalk starts off soft and later hardens – so too the enslavement began “softly” and became harder and bitterer with time (Pesaĥim 39a).
Several Aĥaronim questioned whether we fulfill the obligation of maror with today’s lettuce, which is not bitter at all. Indeed, according to Ĥazon Ish, one may fulfill one’s obligation only with lettuce that has become somewhat bitter. In practice, though, the poskim concur that one may fulfill one’s obligation with lettuce even if it is not bitter, because this was the nature of the bondage: initially the Egyptians enslaved us with gentle words and paid us for our labor, while gradually intensifying the work until it was bitter as gall. The Yerushalmi (Pesaĥim 2:5) likewise tells us that lettuce starts out sweet and then becomes bitter. Thus, it is the custom of all Jews to optimize this mitzva by using lettuce for maror, because maror need not be bitter when it is eaten; rather, it must be of a species that eventually becomes bitter. Some people add a bit of horseradish to their lettuce so that they taste some bitterness.
Since lettuce often contains bugs, it should be cleaned and checked thoroughly on Erev Pesaĥ. Nowadays there are ways to grow bug-free lettuce, and it is best to use such lettuce in order to avoid potentially violating the prohibition against eating bugs.
One must eat a kezayit (half an egg’s volume) of maror, and one may estimate this size by sight. As we have learned, some customarily calculate a kezayit by weight, which is about 29 grams. However, with lettuce there is very little difference between a weight- and volume-based kezayit.
. See Ĥazon Ish OĤ 124, commenting on Pesaĥim 39a. Ridbaz states that lettuce might not be maror at all. Nevertheless, most poskim, including Maharam Halawa and Tashbetz, believe that lettuce is the best type of maror, because the Gemara finds allusions in its name and because it alludes to the enslavement by its very nature – starting off soft and sweet. This is the ruling in SA 473:5, Pri Ĥadash 472:5, and SAH 472:30. Additionally, according to Raavya and Hagahot Maimoniyot (quoted by Tur and SA), whatever is mentioned first in the Mishna is the preferable choice for maror, and lettuce (which the Mishna calls “ĥazeret” even though “ĥazeret” in modern Hebrew is horseradish) is in fact mentioned first. Responsa Ĥazon Ovadia §35 expands on this topic.
We should also add that another opinion appears in a beraita in Pesaĥim 39a: any bitter vegetable that exudes latex and has a blanched complexion (i.e., it is whitish-green) is maror. The Rishonim debated this: according to Ri’az, Maharam Halawa, and others, any vegetable that has these characteristics is indeed maror. According to Smak, on the other hand, these characteristics are just features common to the five types of vegetables mentioned in the Mishna, but no vegetable outside what is listed in the Mishna is acceptable for maror. Rif and Rambam do not mention these characteristics, either because they maintain that only the five vegetables listed in the Mishna are acceptable, and these characteristics consequently are meaningless, or because they maintain that we are not experts in identifying these characteristics, so we have no ability to determine what other vegetables are fit for maror. Practically, one who does not have lettuce or horseradish should use another vegetable that has these characteristics, but should not recite a berakha over it, in case we are not adept at identifying these characteristics or it is not one of the five acceptable species. This is the ruling in MB 473:46 and BHL ad loc.