From what I have read, it was the standard for TV screens back in the '60s when the EXif standard was first formulated. I don't think it applies to either CRT or LCD monitors.
PPI doesn't apply to NTSC TV, for the simple reason that horizontal data is analog, not digital. There are no dots horizontally. Each line is a continuous sweep.
The NTSC standard for vertical resolution is 525 lines interlaced - 262.5 lines in one field, 262.5 in the next. The two fields refresh at a 30 Hz rate.
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I notice that Paint Shop Pro defaults to 180 PPI.
A point nobody has mentioned, that I've noticed: DPI means nothing on an inkjet printer, either. Well, not exactly nothing - but it's fixed. My cheap (but surprisingly high quality) Canon PIXMA 4000 has 1000 DPI, as I recall. That's the number of ink dots per inch.
-js


