When looking at a map that has been rotated 45 degrees, which way is North?

I have tow theories: Reading left to right, North would be upper left (but that would only work for some languages). More likely, using geometry as a starting point, a circle is measure clockwise with 0 degrees pointing straight up and 45 degrees would be upper right. Any other thoughs?

If it's not too much trouble I would like to know the accepted answer and the background about "why" that's the answer.

Thanks in advance from a new member.