I want to mount a board to the wall like in the image. It's currently rectangular, so I need to cut out the blue section.

How do I mark the board correctly before cutting? It's not quite trivial, because the angle Alpha is more than 90 degrees, maybe more like 95 - but I can't measure it. If there's a special tool for that, I don't have it (and if I did have it, I'm not sure it would be worth the effort fiddling with compasses...?). Is there a way of getting the angle right without having to measure it and do a geometric drawing, or would I actually need to get a tool for measuring outward angles (if so, what's that called?).

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3 Answers 3

  • Quick and dirty angle-finder - two straight edges and some tape. Straight cardboard will work. Put them against the walls and tape them together at that angle.
  • Cut a piece of cardboard to fit the angle of the wall. Correct until it's as close as you want. Then apply it to the wood.
  • Or, hold the wood up to the wall. Use any straight-edge to extend the angle onto the wood. Then draw a line parallel to that line where the end of the line at alpha is the correct distance from the end of the board at the measured distance from the window to alpha from the side of the board.

In addition to the more strategic approaches Ecnerwal suggests, I'd offer a technique known to my father as "creeping up on it".

Just notch out a modest bit of the corner at a roughly correct angle, then test fit it. See how it looks. Adjust until you get the angle right, then cut deeper until it fits overall how you wish.


Make a template, cut and glue or securely tape sheets of paper or card so that they make the shape you need.


Buy a profile gauge. or make one from bamboo skewers and corrugated card.

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