I posted the original. Jim, you are right on. As we all learned, when you put glue on something and start clamping, the glue acts as a lubricant. The pieces being glued "slide" in relation to each other. My intent was to start by gluing something that was already a solid 90 degrees.
FYI. By sheer luck, when I picked up the 1st 90 degree moulding at Lowes, I carried it over the to isle with the carpenter squares. IT WAS "OFF" A HAIR! Carried the square with me over to the mouldings to check another one. 2nd one was OK.
After cutting off the top decorative part, I glued on the 2 parts (marked "B"), the "90 degrees" closed up slightly, to -90 degrees. I laid a board across the open end & ran a clamp from that board to the point of the triangle and squeezed it to 90. Achieved perfection.
You guys are right. IF you could clamp the boards to anything that was surfaced to a perfect 90, "A" would not have been needed. I don't have the tools for that and am too D____ old to buy anymore.
jerry