Those of you who still have an old-style CRT television set: look at the edges of the screen. Halfway down, just outside the main viewing area, there is usually a single “pixel” (i.e. a single triplet of red-green-blue phosphor dots).
What’s that all about then?
It’s been a while since I myself looked into this question. I could be wrong, but I seem to recall it having to do with calibrating the scanlines/electron beam.
When the TV is being assembled the three electron guns inside would be made to draw blue, red, and green lines across the middle of the screen and calibrated so the three colours line up with that notch. One some old projection screen TVs you could do the same sort of setup in a menu called “convergence”. There are most likely an entire extra column of pixels off to the side of the main display, but they are masked off an opaque coating inside the glass except where that notch is.
I’ve noticed this notch in the opaque coating on the sides of my one remaining CRT TV. I never knew what it was for. I’d have to look when I get home to verify there is a whole column, but that’s what I’d expect.