grammar - Feeling unsure about the connection of the two sides of the comma

地図は普通、北を上にして掛かれる。 (From line 7: https://www.docdroid.net/847v2dg/img-20170413-0001-new.pdf.html)

=> "The map is normal, it can hang with the north attached to the top."

I feel very unsure about this one since there is no copula and no connector (like for example 地図は普通で), but I can't make sense of it otherwise. I'm also not sure whether the potential form expresses an ability here or makes an objective statement (I think there was something about that with potential form, please correct me if I'm wrong, I also don't feel like I understood it).

Answer

Is there a typo here? Shouldn't 掛かる be 掛けられる ? If so, it's a passive, not a potential. Try analysing the sentence without 普通 .

地図は、 北を上にして掛けられる means "Maps are hung with North at the top".

Then restore the 普通:

地図は普通、 北を上にして掛けられる means "Maps are usually hung with North at the top".

One problem is that we tend to assume that the 、 punctuation mark is like an English comma, marking off a grammatically distinct section of a sentence. So when we come to one we stop to look at the meaning of what precedes it. That's not what it does: it's best to think of it as simply indicating a point in the sentence where if you were reading it aloud you might pause to breathe. I think what's happened here is that you've come to a comma so stopped to make an interim translation of the first part of the sentence as "the map is usual or ordinary" and then gone on to deal with rest of the sentence, not realising that 普通 is in fact an adverb meaning "usually", "ordinarily" and that this is a general statement about the way maps are hung.