In chromaticity charts the CMYK color space is usually four-sided. I've lately had to read quite a few charts and it suddenly dawned to me that I don't really understand why. It has just not been something I've needed to think about until now.
Image 1: A sample CIE31 chromaticity graph showing the spaces*.
One of my working hypotheses is that it is actually a triangle. This triangle just clips for some reason. But there might be other reasons such as non-ideality of the color primaries, or bad plane. So why is it squarish?
* This image is not so clearly a polygon, like it's usually depicted as, but it's the best one I had at hand.