Yesterday evening, I spent time on the inside of the window applying the acrylic around the inside of the opening.  I wanted to make the joints as tight as I could and of course needed a measure of what degree they were.  I couldn’t find a compass.  The internet came to my rescue.  I typed in “obtuse angle” and copied and printed out 120, 130, and 135 degree angles, held up the sheet to one of the window corners and discovered that 135 degrees was the correct measure.  Now, to get the cut, each piece has to have the angle, therefore, I split the measure in half to give 67.5 degrees.  Subtract this from 90 and that gives 22.5 degrees.  This is the measurement to use on the miter saw.  I worked through each piece, starting at the bottom and placing up from there until the entire window was surrounded as I desired.  The side pieces didn’t behave as much as desired so I put a small board there to make them behave.  This morning, the sun was shining, the glue was dried and I removed the board to admire yesterday’s efforts.  While standing there, it occurred to me that one could easily superimpose a box over the window so that the top and sides corresponded with the box.  In that scenario, each of the corners are simply right triangles.  The right triangle has three angles: 45, 45, and 90.  The angles were really easy to calculate in that manner without going through all the printing and  comparing that I actually did.  Oh well.  They are there.   The job is done.