How to Miter Corners
Mitred corners can be a problem for beginning needepoint. For years I hated having to figure out the correct diagonal line which would make the design meet perfectly. And a border design which went around a corner — forget it (I still don’t like these AT ALL!). But sometimes you cannot avoid having to make two areas of stitching meet along a diagonal line. Probably Four-way Bargello is the most noticeable example of this, in this there are diagonal intersections all over the place.
Mitered corners have the two sides meeting along a diagonal line, for example the way the corners of a picture frame look. In needlepoint, you will often find that you need to turn a corner when you are using straight stitches like Gobelin. This is when you miter.
To miter, first stitch the last complete stitch on either side of the corner, providing your basis for the diagonal. Now make each subsequent stitch one thread shorter than it neighbor. Do this for both sides of the corner. As the stitches get shorter, you will notice that they are meeting along a diagonal line going up the center of the corner. The last pair of stitches you make should cover one thread each.

Mitered Corner, copyright Napa Needlepoint
That’s it! Your corner is properly mitred. Now if you want to accent the corner a bit, or if you really do not like the empty hole at the very corner, make a long diagonal stitch from the empty hole on the outside to the hole where the two full stitches meet.These may be left out but having them makes the corner look more finished.
Now even those who are just beginning needlepoint can make perectly mitered coners.