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And 21:9 is for consuming "cinematic" video, or two 4:3-ish productivity apps side-by-side, with some space left over.

I prefer 21:9 because I feel like it most closely matches my natural field of view.

I enjoy my 34" 3440 x 1440, but I think that a 32" (or slightly larger) 4K might ultimately be the better choice while still fitting in my field of view.

I think the 5k monitor Apple was selling for a while was the same idea - you can fit 4k video pixel for pixel and also the editing tools.

16:10 is A4 paper.

Metric paper sizes have the ratio sqrt(2), 1.414.

(A0 has area 1m². Halving the sheet gets the next size. This works very nicely for scaling documents smaller or larger.)

16:10 is closer to Legal (8.5" × 14") than A4 (which is 210 × 297 mm)