@dutchstandardalphabets @eWalthert It's all cost driven. You can nail cheap, common straight boards together to make a square frame in just a few minutes and reuse it again on job after job. It would take hours to build a custom curved frame by sawing lengthwise curves in costly wide boards, and then you throw them in the waste pile because it's so unlikely you will need that exact radius curve again, and you can't afford to warehouse them all.