Things You'll Need
Adjustable angle square
Half the trick of putting up a great-looking chair rail is getting the cuts right. Because the chair rail runs right across the middle of the wall, any gaps or other problems in the installation will be more easily noticed than on your floor trim or ceiling molding. Power miter saws give you the ability to make the exact angled cuts you need. Even with this tool, though, you will have to make some adjustments for the inexact nature of most walls.
Use your adjustable angle square to measure the angle of inside corners, pressing the tool into the corner and taking the reading. Divide the reading by 2 to get the correct angles for each cut. For example, if the corner is 86 degrees, each of your cuts will be 43 degrees.
Set your miter at the correct angle. Set the first piece of chair rail on the saw platform, with the piece standing on its bottom edge (the way it will go on the wall), with the angle making the front of the trim shorter than the back. Turn the miter to the same angle on the other side of the platform, and cut the second corner piece in the same fashion.
For your exterior corners, make 45-degree cuts with the faces of the boards longer than the backs. Exterior corners tend to be easier to work with, and unless it's unusually far out of line, you shouldn't have to adjust the angles of the cuts to get them to fit.
For long walls that require more than one piece of chair rail to span them, put opposing 45-degree cuts on the ends where they meet, cutting toward the face on one board and toward the back on the other, so one slightly overlaps the other on the wall. This will provide a tighter and more seamless fit then butting the straight ends of the boards together.
If you're unsure of your inner corner angles because the wall is especially wavy and hard to measure, use some scrap chair rail to make test cuts, fitting the pieces together in the corner and adjusting the miter angle until you get it right.
Wear protective eye gear when using power tools.