If you're talking about trim carpentry/etc, you can do this on inside or outside corners with something as simple as a sliding bevel gauge
https://bowsaw.files.wordpress.com/2010/03/bevel1.jpg and some thin and wide painters tape.
* Place one strip of thin tape carefully parallel to each wall
* Place one strip of wide tape beside each thin strip, make sure they cross each other
* Pull up the thin tape
* Draw a line between the intersection points where the wide strips cross
* Use the sliding bevel gauge to establish angle between the drawn line and the edge of the tape
* Flip gauge to check
* Cuss and adjust as necessary
I cheat a little making sure the angles are just a very little tighter so that the back side of the trim barely doesn't touch in case I need to fine tune the cuts with a rasp.