It seems a horizontal guide is 0° and a vertical 90°.
But there are positive degree values and negative.
Does this depend on, from which "hemisphere" i create the guide ?

Looks like it, yes. Not sure if this is a bug or just a quirk, maybe it would be worth making this a bug report.
You can look through the existing ones here:
https://bugs.launchpad.net/inkscape

and if there isn't one, you could just ask the devs there

It seems all in order to me. All vector spaces must use a coordinate system, and in Inkscape the document area you see outlined is the first quadrant of the Cartesian coordinate system (you might recognize this from maths classes, the top right area).

This means that the origin (0,0) is in the bottom left corner, just like in school

And that's how guides angles are handled, they are straight lines, if parallel with X axis the angle is 0, if vertical (parallel with Y axis) the angle is 90°, if you go below in the 4th quadrant (clockwise) you count negative degrees.

A small maths reminder (from the internet):

@prkos: Double-click on a guide to see what Raspi means. Turn it a bit, double-click again. Grab it on different ends (in relation to its origin). You'll see that the guide angle is sometimes denoted as positive and sometimes as a negative value, even if the direction of the guide, and also which end faces which part of the drawing, is actually the same. Whether the value is positive or negative depends upon the end you grab to rotate it/the direction you rotate to, not upon its actual angle (which imo should be the same, no matter how you got there).

(or well, maybe the Raspi didn't mean this at all...)

Ah now I see what you mean. If I rotate the guide counter-clockwise a bit (with Shift) so it ends up across second and fourth quadrant (approximately) it gives negative values but only if I grabbed it below the origin. If I grab north of origin and move the same way the angle is positive.

I'm guessing it might have to do with where the mouse pointer is positioned when released after rotating. Is this a good thing?

Moini wrote:(or well, maybe the Raspi didn't mean this at all...)

its exactly what i meant, Moni

prkos wrote: Is this a good thing?

Its mostly confusing.
The issue came up when i tried to construct an arrowhead with the help of guides and setting specific angles was, well confusing.
It seemed certain degrees values are missing. And i didnt know if its a bug or a feature.

I don't know either - but I'd like to Can you look through the bugs / make a report, Raspi?

Moini wrote:I don't know either - but I'd like to Can you look through the bugs / make a report, Raspi?

Not really tbh, and i dont know how to word it.

Yes, I agree, that's a bit difficult here, if you want to make it short, so people will read and understand...

@prkos: Could you? Btw. it also changes depending on the rotation direction.
E.g. start with a guide which is pointing at 4 o'clock with one 'end' (it's infinite, I know...). Hold the end that is where the number of the clock would be. Rotate it downwards to 5 o'clock, note position. Rotate it downwards to 6 o'clock. Rotate back upwards to 5 o'clock, note different position from first time it was at 5 o'clock. Always grab the same 'end'.

Yes of course, I'll post it. I couldn't find any existing bug similar to this. I'll investigate a bit more for a more detailed report and post the link here when I'm done.

Here we go:
Normalize guides rotation angles to positive values
https://bugs.launchpad.net/inkscape/+bug/1636154

Thank you!!!