Digital system field/frame rates.

[ppppenguin]

HD on computers predates TV HD and 16:9 only arrived as a cost reduction to Computer monitors MUCH later.

HD transmissions to people's homes happened much later but the 16:9 standard for HDTV along with the 1920x1080 image format was developed in the 1980s. The 16:9 aspect ratio was came from work done by NHK Japan to find the subjectively most pleasing aspect ratio. SMPTE 260M, which defines the 1980x1080 60Hz format was first published in 1992. There were very few widescreen displays available back then and they were very expensive professional CRT monitors.

I've read the NHK papers on aspect ratio and think they are dubious at best, but that's what we ended up with. Everybody agreed 16:9 very early on in the days of HDTV when it was still largely analogue. I'm sure I have the NHK papers somewhere but finding them is another matter.

I think the first widescreen computer displays were in the early 2000s.

HDTV had square pixels because anyone who was doing image processing was fed up with the non-sqaure pixels of SD digital. Differently non-square in the 525 and 625 versions too.