Learn the גמרא דף ח: “אמר רב כהנא בר תחליפא” until “איבעיא להו מהו להשתמש תחת הקורה
Learn תוס’ “מניח
Questions:
Why does Rashi and Tosfos both explain according to רב כהנא it is permitted to carry opposite the long wall?
In the question of Tosfos why does tosfos understand it’s considered as there are only two walls ?
Tosfos gives two answers.
What’s the difference between the two answers?
Learn ריטב”א – note how he answers Tosfos question.
https://hebrewbooks.org/pdfpager.aspx?req=37287&st=&pgnum=13
What is the difference between Tosfos and the Ritva answers?
Rava argues and holds the Korah must be placed straight and not on an angle. The gemara says the reason is because a Korah on an angle is not a היכר.
Why doesn’t the gemara simply say the reason is קורה משום היכר and therefore if place the Korah on an angle from the short side to the long side you will only have two walls?
See ביאור הלכה ס’ שס”ג סע’ ל “אינו מניח
https://hebrewbooks.org/pdfpager.aspx?req=60389&st=&pgnum=66