Siman - Eruvin Daf 3

• The korah serves as a heker

Rav stated on Daf 2a that the Chachomim who hold that the beam of a mavoi may not be higher than twenty amos, derive their ruling from the height of the entrance into the Heichal, which was twenty amos. In today's daf, Rav Nachman bar Yitzchak disagrees and says that the reason the Chochomim limit a korah’s height to twenty amos is משום היכרא - to serve as reminder to differentiate between a mavoi and a reshus harabim.

Rashi explains that people will see it and know that the Rabbanon required it, and they will not come to confuse the mavoi with a reshus harabim. It also explains why the Rabbanon held that an אמלתרא - a prominence , above twenty amos is permitted, since people will notice the beam because of it.

• When part of korah or s’chach is above twenty amos

The Gemara asks what the halacha is if part of the korah is within twenty amos of the ground and the top part of the korah is above twenty amos? Similarly, what is the halacha if part of the s’chach is within twenty amos and part of the s’chach is above twenty amos. Rabbah answered, במבוי כשר בסוכה פסול – the korah of the mavoi is valid and the sukkah is passul.

• Rava from the town of Parzakya explained the difference between the korah and sukkah. Technically the sukkah should be valid if the lower part of the s’chach is within twenty amos, except that since a sukkah is meant for an individual, the person sitting in the sukkah will not be mindful and notice if some of the lower s’chach is removed and thereby make the sukkah passul. In the case of a mavoi, however, since it is meant for the public, they will remind each other to fix it if the lower part of the korah disintegrates.

• Ravina said that Rabbah was machmir by the sukkah since it is a d’Oraysa, and lenient with the mavoi since it is a d’Rabbanon. 

The Gemara brings an alternative version of Rabbah in which he rules, במבוי פסול בסוכה כשירה – the korah of the mavoi is passul and the sukkah is valid, with explanations for Rabbah’s rationale.

• The size of an amah

Abaye said in the name of Rav Nachman, אמת סוכה ואמת מבוי באמה בת חמשה – the amah-unit mentioned in the laws of sukkah and a mavoi is a five tefachim amah unit, and אמת כלאים באמה בת ששה – the amah-unit mentioned in the laws of kilayim is six tefachim. Rashi explains that Rav Nachman is stating that the amah-unit that will result in a chumrah is always used. This must be Rav Nachman’s intent, otherwise how would he apply different measures when the Mishnah does not indicate which measure should be used.

Rava said in the name of Rav Nachman that all amah-units are six tefachim, אלא הללו שוחקות והללו עצבות – the amos for kilayim are loose amos, meaning measured with a loose fist, and the amos for a sukkah and a mavoi are measured with tight amos, meaning measured with a tight fist . Tight fists are called “sad” resembling the compressed lips of someone who is sad. Loose fists are called “happy” resembling the slightly parted lips of one who smiles.