The first Mishna in Massekhet Eiruvin taught that a Mavoy (alleyway) whose walls are taller than 20 amot (cubits) or more than ten amot apart cannot be permitted by use of the ordinary heker (reminder) of a Lehi (side post) or a Kora (cross beam) (see 2a-b).
At the same time, the Mishna taught that a tzurat ha-petah – a symbolic doorframe – will suffice to close the open end of a Mavoy even if it is wider than ten amot. We also learned a baraita (3a) which ruled that an Amaltera – a decoration above the entranceway – will allow a kora to work even above 20 amot.
Now the Gemara asks whether these two methods can be switched. Will a symbolic doorframe permit carrying in the Mavoy even if it is higher than 20 amot? Will an Amaltera allow carrying in a Mavoy even if the opening is wider than ten amot?
A close reading of the Mishna convinces the Gemara that each of these special conditions will only work in the specific case where it is suggested by the Mishna. Nevertheless, the Gemara’s thought that we could, perhaps, apply them deserves some explanation.
With regard to the Amaltera, Rabbi Yaakov Kahane, in his Gaon Yaakov, posits that the Gemara’s suggestion is based on its quandary about the basis for the exception of the Amaltera in the case of walls higher than 20 . Two possibilities are:
The fact that it looks unusual and draws attention.
The fact that the importance of the decoration elevates the opening to be considered a door.
According to the first explanation, while people will notice something out-of-the-ordinary that is higher than usual, if the opening is very wide, it will be less noticeable and will not accomplish its purpose. If, on the other hand, the issue is that the opening becomes significant as a door by dint of the important decoration, the Amaltera should succeed in accomplishing that.
According to the Gemara’s conclusion, it appears that the first explanation of the Amaltera appears to be the correct one.