When installing crown molding, I find it difficult to measure accurately between the corners of a room. A tape measure can't measure precisely enough into corners for me to cut nice, tight miter joints.
To record this tricky measurement accurately, I made a pair of molding jigs like the one shown at top. On each, I cut one end to 90° and the other to 45°, with the longer edge exactly 6" long. (I made the second jig with the angles switched to fit together in a corner as shown.)
Then, I attached a couple of triangular cleats, cut from 3/4" stock, to match the spring angle (the angle between the wall and the back of the molding). Because walls and ceilings seldom meet at a true right angle, I clipped the corner off the 90° angle of the cleat so it wouldn't interfere.
To measure wall length, I simply butt these two jigs together in each corner of the room and mark the walls at the end of each jig. I then measure between the marks, add 12", and cut the molding to that length. The jigs also are handy for marking reference lines every few feet along the wall to keep the molding aligned while I fasten it in place.
--Jeff Jones, Bakersfield, Calif.