Cornice and coving corners
Most ranges of cornice and coving include preformed corners. With coving these are usually simply preformed internal and external corners. Corners for patterned cornices are often features in there own right so that the lengths of cornice simply butt squarely up to the corner piece - this has the advantage that there's no need to match up the pattern at the corner.
With simple coving, it's fairly easy to cut your own mitres as detailed below.
Cutting in a mitre box
Use a mitre box to accurately cut the corner mitres, make sure that the mitre box is large enough so that the two edges of the coving can fit flat against the bottom and side of the box (see right).
When using the mite box, always place the ceiling edge of each piece of coving at the bottom of the box, with the wall edge against the side towards you - on lengths requiring mitres at both ends, it helps to pencil a mark on the ceiling edge of the coving to ease identifying it when each end is cut.
Transfer wall measurements to the wall edge of the coving and carefully line this up in the mitre box.
Support lengths of plaster and gypsum cornice/coving while they are being sawn - they have some degree of flexibility but can easily break under their own weight.
Take care that you are cutting the angle in the right direction, it's very easy to get confused.
The five basic cuts are shown to the right.
Use a fine toothed saw to cut the cornice/coving .
Smooth off the cut ends using fine glass paper.