Eiruvin 1:1-2

Eiruvin 1:1

If the entrance to an alleyway is more than 20 amos high (about 30 feet), it must be lowered; Rabbi Yehuda says that this is unnecessary. If the entrance is wider than 10 amos (about 15 feet), it must be reduced. If the entrance is in the shape of a doorway (with a pole on either side and a crossbar on top), it is not necessary to reduce it even if it is wider than 10 amos.

Eiruvin 1:2

Beis Shammai says that, in order to render an alleyway fit to carry within, the entrance must have a vertical pole on the side and a horizontal beam across the top. Beis Hillel say that either a pole or a beam is sufficient; Rabbi Eliezer says two poles. A student said in the name of Rabbi Yishmael in the presence of Rabbi Akiva that Beis Shammai and Beis Hillel did not disagree in the case of an alley that is narrower than four amos (about six feet); in such a case, either a pole or a beam is sufficient. They only differed in the case of an alley from four to ten amos wide. In such a case, Beis Shammai say that it requires a pole and a beam while Beis Hillel say a pole or a beam. Rabbi Akiva said that Beis Shammai and Beis Hillel disagreed even in the case of an alley narrower than four amos.