OPINIONS: The Gemara describes a Revi'is (which is a measure of liquid volume) in terms of a cubic Etzba, or thumb-breadth (which is a measure of length). Rav Chisda explains that a Revi'is is equal to the volume contained within a box that is 2 Etzba'os long, 2 Etzba'os wide, and 2.7 Etzba'os high (2 X 2 X 2.7 cubic Etzba'os, or 10.8 cubic Etzba'os).

The Rashbam explains further that a Revi'is is equal to the size of 1.5 average eggs.

These definitions have practical ramifications for every Mitzvah that requires a Shi'ur of a Revi'is, Beitzah, or k'Zayis (half of a Beitzah), and they are the subject of much discussion among the Acharonim.

(a) The NODA B'YEHUDAH (see TZELACH here and 116b) used his thumbs (Etzba'os) to determine the volume of an egg, based on the figure that the Chachamim gave for the size of an egg in terms of thumb-breadths. He then measured the volume of an average-sized egg and found that it was only half of the volume that he calculated with his thumbs. He deduced that either thumbs had become larger than they were in the times of the Gemara, or eggs had shrunk. He argued that it is illogical to presume that our thumbs are larger than those of generations past, because it is known that each generation is weaker than the previous one. Rather, he concluded, it must be that eggs have become smaller.

Therefore, for any Mitzvah which involves the Shi'ur of a Beitzah (or Revi'is), one should use twice the amount that the Gemara requires. (For example, since the Gemara says that one must eat "one Beitzah" of Matzah on the first night of Pesach, today one must eat two Beitzim of Matzah, based on today's average egg size, in order to compensate for the decrease in size of eggs.)

This opinion is cited as the Halachah by the CHASAM SOFER (Teshuvos OC 127), the VILNA GA'ON (Ma'aseh Rav #105), and the CHAZON ISH (Kuntrus ha'Shi'urim, OC 39).

The Chazon Ish converted the size of a Revi'is into cubic centimeters. Based on the findings of the Noda b'Yehudah, who judged the value of an average thumb-width to be 2.4 centimeters, the Chazon Ish defined a Revi'is as 150 cc (cubic centimeters, or, in terms of weight, approximately 150 grams of water).

(The TESHUVAH ME'AHAVAH, a disciple of the Noda b'Yehudah, rejected his teacher's calculations based on the fact that "the Noda b'Yehudah was one of the tallest men in the generation, and he measured with his own thumbs." The Chasam Sofer rejects this argument and points out that although 2.4 centimeters is a relatively large width of a thumb, it is not uncommon for thumbs to be that size. See Midos u'Mishkalos Shel Torah, chapter 87.)

However, these calculations are problematic for several reasons.

1. The RAMBAM (in Perush ha'Mishnayos to Eduyos 1:2) writes that a Revi'is equals the weight of approximately 27 dirhams (a common coin in Arabian countries at the time of the Rambam, and still used today in some places). Based on numismatic records and collectors' dirhams, experts have determined that the weight of 27 dirhams equals approximately half of the size of the Noda b'Yehudah's Revi'is.

2. Additionally, the Rambam (Hilchos Eruvin 1:12) writes that a Revi'is weighs as much as 17.5 Dinars (common coins used in the times of the Gemara). The weight of the Dinar to which the Rambam refers is well known, because the Rambam based his measurements on the BEHAG and the RIF (Kidushin 12a) who write that the Dinar in the times of the Gemara was equivalent to the contemporary Arabic "Sheshdang" Dinar. Furthermore, the Rambam himself specifies the weight of a Dinar in terms of barley grains, which later Rishonim compare to carob pits (four barley grains are equal to one carob pit). These modes of measurement correlate with precision even today, and thus the weight of the Dinar is fairly well known. (That is, it is highly unlikely that both the barley grains and the carob pits shrank equally since the days of the Rishonim.) The size of a Revi'is as calculated based on the weight of the Dinar equals, again, half the size of the Noda b'Yehudah's Revi'is.

3. The Mishnah in Kelim (17:11) says that the measures of volume used by the Chachamim were the same as the Italian (Roman) measures. Based on comparisons to ancient Roman measures, it can be demonstrated that the size of a Revi'is was much smaller than the size proposed by the Noda b'Yehudah (see Midos u'Mishkalos Shel Torah 1:48).

4. The CHAFETZ CHAIM in BI'UR HALACHAH (OC 271:13, DH Shel Revi'is) points out that the Gemara in Yoma (80a) says that a person can hold more than a Revi'is (or the volume of 1.5 eggs) in both of his cheeks at one time. The Chafetz Chaim writes that after considerable experimentation, he observed that the average person can hold, at most, the volume of two modern eggs in his mouth at once. According to the Noda b'Yehudah, who says that a Revi'is contains twice the amount of eggs than it did in the times of the Gemara, a person should be able to hold at least three modern eggs in his mouth at once. No average-sized person is able to do this.

(The TOSFOS RID in Yoma (80a) in fact preceded the Chafetz Chaim with this observation. He writes that even 1.5 eggs cannot be held in the cheeks at once unless a person holds his head downwards, in an unnatural position, in order to prevent himself from swallowing the liquid in his cheeks. See Insights to Yoma 80:1.)

5. The CHAZON ISH (as recorded in the Steipler Ga'on's SHI'URIM SHEL TORAH 3:9-10) raises another question on the measurement of the Noda b'Yehudah. Throughout the generations, the Jewish people have had the custom to use between 70 and 100 grams of silver for the Mitzvah of Pidyon ha'Ben, the redemption of the firstborn son. The Rambam writes that a Revi'is weighs the same as 17.5 Dinars (see 2. above), and it is known that Pidyon ha'Ben is performed with 20 Dinars (or 14% more than the weight of a Revi'is). Accordingly, the weight of the silver used for Pidyon ha'Ben should be 14% more than the weight of silver which is equivalent to a Revi'is. However, if a Revi'is is 150 grams (as the Chazon Ish calculates based on the Noda b'Yehudah), then one should use 14% more of that amount -- 150 grams -- of silver for Pidyon ha'Ben, or about 170 grams, but it was never the custom anywhere to use that much silver for Pidyon ha'Ben.

6. It is interesting to note that some ancient eggs actually survived to present times. RAV YAKOV GERSHON WEISS (author of MIDOS U'MISHKALOS SHEL TORAH) related to us that mummified eggs found in the pyramids of Egypt, as well as eggs preserved by the ashes of Vesuvius in the ruins of Pompei, are essentially the same size as today's eggs.

(b) RAV CHAIM NA'EH (in SHI'UREI TORAH) lived in Eretz Yisrael during the time of the British Mandate. He measured the Revi'is based on the dirham which was used in Israel during the reign of the Ottoman Empire, and was still in use during the British Mandate. The dirham he measured weighed 3.2 grams. Based on the Rambam's assertion that there are 27 dirhams in a Revi'is, Rav Chaim Na'eh concluded that the Revi'is must be 86.4 grams (27 X 3.2 = 86.4). (He pointed out that 86 happens to be the Gematriya of "Kos," cup.) According to this figure, the width of an Etzba, if one calculates backwards, is 2 centimeters, which is closer to the average thumb-width.

However, this size of the Revi'is is also problematic.

1. The weight of old dirhams fluctuates between 2.7 and 3 grams. Hence, the maximum size of a Revi'is should be 81 grams, and not 86 as Rav Chaim Na'eh states, who measured the Revi'is based on a dirham of 3.2 grams.

2. Second, records of the "Sheshdang" Dinar show that it weighed 4.25 grams, which makes a Revi'is (which is 17.5 Dinars) about 74.4 grams. When divided by 27, the weight of a dirham should be 2.75 grams, which fits within the range of the average weight of old dirhams.

3. Third, according to Rav Chaim Na'eh, an egg today should have an average size of 57 1/3 grams. (An egg is 2/3 of a Revi'is: 2/3 X 86 = 57 1/3.) In reality, the average egg is somewhat smaller than that. According to the calculation of the Revi'is based on old dirhams, the average egg should be about 50 grams, which is much closer to the actual size of today's eggs.

4. Fourth, there are 24 Etzba'os in one Amah. The size of an Amah is the distance from the elbow until the tip of the middle finger of the average arm. If the average Etzba is 2 centimeters, as Rav Chaim Na'eh asserts, and there are 24 Etzba'os in an Amah, then the Amah should be 48 centimeters. However, the length of the average arm, from elbow to tip of middle finger is less than 48 centimeters. Using the measure of the Etzba based on the Revi'is as calculated by dirhams, the Amah comes out to 46 centimeters, which is closer to the length of the average arm.

5. Fifth, RAV SHLOMO ZALMAN AUERBACH zt'l points out that the Gemara in Shabbos (14a) teaches that the Rabanan decreed that a person who drinks liquids which are Tamei becomes Tamei. The Rabanan enacted that Gezeirah so that a person would not eat Terumah at the same time that he has a Revi'is of Tamei liquid in his mouth (see Tosfos in Shabbos there). He remarks that according to Rav Chaim Na'eh, one could not possibly fit any food into one's mouth when there is already a Revi'is of liquid there.

(c) A third opinion suggests an even smaller calculation of the size of a Revi'is than Rav Chaim Na'eh's figure: approximately 74 cc. This opinion bases the size of the Revi'is on the size of the dirham at the time of the Rambam. Historical records show that the dirham has increased in weight through the centuries, and Rav Chaim Na'eh's dirham was larger than the Rambam's. (See MIDOS U'MISHKELOS SHEL TORAH, chapters 60-64.)

This figure is consistent with both the weight of the Dinar and the size of common eggs. The only problem that remains is the size of thumb-widths. If a Revi'is is 74 cc, then the average thumb-width should be 1.9 cm. However, the width of 2 cm is much more common. It is possible that the people of earlier generations had smaller fingers, or that when they measured finger-breadths, they pressed their fingers closer and harder together.

OPINIONS: The Gemara discusses at length the consequences and concerns of one who performs certain actions in pairs (for example, he eats or drinks a multiple of two of any item), and how this could give certain power to the Shedim to inflict harm.

What is the basis for the Gemara's concern for Zugos?

The Acharonim propose different rationalizations for the phenomenon of "Zugos," pairs.

(a) The SEFER MALKIEL teaches that the existence of Shedim represents the forces of nature through which Hash-m punishes those who demonstrate a lack of faith in Him. Hash-m is the ultimate and only One, the pure Singularity in the world. When a person performs an act of Zugos, or "multiplicity," he gives the Shedim a foothold because he demonstrates a lack of Oneness in the world. When a person eats an odd number such as three, the even numbers pair, cancel each other, and leave a singular "one." That is why the Shedim have no power over odd numbers.

(b) The MAHARAL explains why the Shedim dominate wherever there is a lack of Yichud Hash-m. (See Gevuros Hash-m 38; Be'er ha'Golah, Be'er ha'Sheni, p. 28; Nesivos Olam, end of Nesiv ha'Avodah.)

The Maharal relates that the Shedim are a secondary consequence of the creation of the world. When Hash-m created the world, an outgrowth was the development of Shedim. The Shedim were not meant to be the purpose of creation, like the other creatures of the world. Rather, they were a corollary of creation, and they came into being since the world would not be complete without them. Since their whole existence is secondary, they have a less-direct connection to Hash-m, and, consequently, they have power over anything that is secondary and is less directly connected to Hash-m.

(c) The RAMBAM in Moreh Nevuchim writes that most Mitzvos that seem to lack purpose can be contrasted to the practice of early idolaters. In this vein, the following explanation may be suggested:

One of the basic tenets of the belief of the polytheists, who believed that there were many gods, was that all of the gods stemmed from two basic powers, a force of good and a force of evil (Sanhedrin 39a). For this reason, the idolaters had a practice to be careful to eat two of every item to satisfy the forces of both good and evil. To counter this conduct and ideology, the Rabanan decreed that the Jews should be careful not to eat two of any item, and certainly not to do so intentionally, because doing so demonstrates a trace of idolatry for which a person will be punished. (M. KORNFELD)

OPINIONS: The Gemara discusses at length the consequences and concerns of one who performs certain actions in pairs (for example, he eats or drinks a multiple of two of any item), and how this could give certain power to the Shedim to inflict harm. (See previous Insight.)

What is the Halachah in practice today? Do the stringencies of Zugos apply today?

(a) The RASHBAM (110a, DH Rabah) writes that as time passed, the concern for Zugos decreased. (According to the third explanation in the previous Insight, this is particularly logical, because no one today remembers how the idolaters used to conduct themselves.) Even in the times of the Amora'im, the concern for pairs had decreased, as the Rashbam explains.

TOSFOS (Yoma 77b and Chulin 107b) explains that during his time there was no longer any concern for Zugos. He explains that apparently the forces that had power over Zugos have become so diminished that they are extremely rare and do not warrant any concern.

(b) The TUR (OC 170), however, makes a short remark that one should not eat or drink anything in pairs. The Tur (OC 183) also mentions that a person should not recite Birkas ha'Mazon after he drinks two cups of wine during the meal, because he will be so worried that Shedim might harm him that he will not be able to concentrate on the blessings. The BEIS YOSEF there writes that the Tur should have omitted this Halachah because Tosfos writes that the Shedim are no longer prevalent, and in the SHULCHAN ARUCH the Beis Yosef indeed makes no mention of it. The Acharonim suggest various answers as to why the Tur mentions Zugos at all.

Perhaps the Tur mentions the Halachah of Zugos with regard to Birkas ha'Mazon because it is a matter of one's ability to concentrate on the blessings. Even though, in reality, there is no longer any concern for Shedim, some people are still worried about them. Such a person's mind will not be clear and calm enough to concentrate when he recites Birkas ha'Mazon. Therefore, the Tur says that he should not recite Birkas ha'Mazon after he drinks two cups.

Why, though, does the Tur mention that a person should not eat or drink in pairs?

Perhaps the Tur maintains that even though the Shedim are no longer prevalent nowadays, and one does not need to be concerned with Zugos in his everyday conduct, the Tur cautions against enacting a decree or introducing a custom that involves pairs. The reason for this is twofold. First, perhaps the Shedim will return. Second, the Gemara (110b) says that even when the Shedim have no power, Kishuf (witchcraft) still has power over Zugos. Someone who eats Zugos is more vulnerable to be harmed by Kishuf. Although Kishuf is rare, a decree or custom that will put a person in danger of Kishuf should be avoided.

QUESTION: The Gemara asks how the Rabanan could have instituted the Mitzvah of four cups, if the drinking of pairs poses a danger (see previous Insights). The Amora'im offer three answers: the Seder night is a night of special protection from harm; the cup of Birkas ha'Mazon (the fourth cup) adds to the other cups only goodness and not danger; and each of the four cups is a separate Mitzvah.

None of these reasons justifies another enactment of the Rabanan that involves pairs: the requirement to recite "ha'Motzi" over Lechem Mishneh, two loaves of Chalah, at each Shabbos meal. Why were the Rabanan not concerned for Zugos when they enacted the Mitzvah of Lechem Mishneh?

(a) When the Rabanan enacted the Mitzvah of Arba Kosos, they enacted that one must drink the entire cup (a majority of the cup is considered like the entire cup). In contrast, when they enacted the Mitzvah of Lechem Mishneh, they made no requirement to eat two entire loaves of bread. Since the Mitzvah does not require one to eat a pair of loaves, but only to recite the blessing over a pair of loaves, Lechem Mishneh is not considered Zugos. (Y. SHAW)

(b) The Gemara later (110b) says that loaves of bread are not subject to the concern of Zugos because they are "completed in the hands of man." (However, some old texts of the Gemara, including the text of Rabeinu Chananel, do not include the word "Kikaros," loaves.)

(c) The MAHARSHAM cites the Zohar (Parshas Ekev, p. 273a) that asks this question. The Zohar answers that Shedim do not have dominion on Shabbos (although they apparently do have dominion on Yom Tov, as is evident from the Gemara's question concerning the Arba Kosos).

RAV YAKOV EMDEN asks that the Gemara later (112b) concludes that Shedim are more prevalent on Friday night than during the rest of the week. It is for this reason that the verses normally recited before Shemoneh Esreh (in Chutz la'Aretz) during Ma'ariv are not recited on Friday night; their omission enables everyone to finish Ma'ariv early and return home together while the lamps are still lit (TUR OC 267, in the name of the Ge'onim). How is the Gemara there to be reconciled with the Zohar, which says that the Shedim have no power on Shabbos?

Rav Yakov Emden answers that the Zohar itself asks this question (Parshas Vayakhel, p. 205a). The Zohar answers that the public has nothing to fear on Shabbos. Individuals, though, could be affected by the forces of the Shedim on Shabbos, and thus on Friday night an individual should not go out alone. Since the Mitzvah of Lechem Mishneh was enacted for everyone, it is considered an enactment for the public, and thus there is no concern for Shedim.