What is this なければならない doing there?


では、なぜ北が上 **でなければならない** のだろうか。 (From line 7:
<https://www.docdroid.net/847v2dg/img-20170413-0001-new.pdf.html>) で should be
a particle here right? If so, why is なければならない not attached to any verb? I
don't know what this means.

I would say that the sentence means the following: "Well, why must the north
be attached on top/in the upper section?" But I can't say why なければならない floats
around freely like this.




Here is the passage:

>
> 地図は普通、北を上にして[描]{か}かれる。では、なぜ北が上でなければならないのだろうか。これは地図に経緯度が使われるようになってからの習慣であって、古い地図には南が上のものも多い。

In fact, there is a verb. This verb is である. And the negative of である is でない.
_Must be_ can then be written でなければならない.

> Usually on a map, north is at the top. But, how come that north has to be at
> the top? In fact, it has been customary since the use of latitude and
> longitude and it is not unusual to see south at the top on old maps.




Here is the passage:

>
> 地図は普通、北を上にして[描]{か}かれる。では、なぜ北が上でなければならないのだろうか。これは地図に経緯度が使われるようになってからの習慣であって、古い地図には南が上のものも多い。

In fact, there is a verb. This verb is である. And the negative of である is でない.
_Must be_ can then be written でなければならない.

> Usually on a map, north is at the top. But, how come that north has to be at
> the top? In fact, it has been customary since the use of latitude and
> longitude and it is not unusual to see south at the top on old maps.

