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When I check the properties of a photo, the resolution is shown as 72x72 most of the time:
But as far as I know, the resolution depends on the image dimensions (5184×3456 in this case) and also the print size. For example the resolution of an image with 5184×3456 pixels printed on a 6x4 inch paper will be 864 ppi. And in this example, 72 ppi is achieved if we print the photo on a paper of size 72x48 inch!
So my question is, where does this 72x72 resolution come from?
72 units per inch is the definition of the point, which is the basic unit of measurement in the PostScript page description language.
It was based on a unit of the same name used in traditional typesetting, with its origins in the 16th century, but standardized in the 19th century to be 1/72.27th of an inch. (The extra fraction came about to accommodate the size of existing hot metal type.)
The first Apple Mac had a screen resolution of 72 ppi (pixels per inch). With the adoption of PostScript on the Mac, this meant that 1 point on the paper equalled 1 pixel on the screen. Given Steve Jobs' interest in typography and attention to detail, this was likely not a coincidence, but an intent.
72 dpi is the 'default' resolution, because it means 1 pixel per point.
Some image file formats store a nominal resolution within their metadata. Of course, you can size the image to any dimensions, and the true resolution will mathematically correspond.
72 pixels per inch is not a part of the image, it's an embedded number which is used in the program which you use to watch or edit the image as a guidance how big the image actually is as inches. The number tells nothing of the quality of the image, it's only number.
Program and devices which are used to generate or edit images insert often just that 72 ppi as a default value if nobody programs something other value to be used instead. Image watching and editing programs calculate the image dimensions as inches or millimeters from it and the pixel dimensions.
72 ppi is the common default value, beacause a long time it was close the usual pixel density (=resolution) of common (see NOTE1) computer screens, but in photo editing programs GIMP, Photoshop, etc. one can easily change that number to some other. I change it often to 300 ppi because that is common resolution for printed images and I can see the printed size easily in layout programs. Any other number is as acceptable, only the pixel dimensions mean something for how sharp an image actually can be and how much information it can contain.
WARNING: If you decide to change the ppi-number you can easily spoil your image if you accidentally also allow the program to resample the image to other pixel dimensions. So, do NOT allow resampling if you decide to change the ppi-number, but want to keep the original pixel dimensions and the true image quality intact.
NOTE1: Actually it was state of the art 35 years ago and it was still common 10 years ago. Today in 2022 many people have much denser screens.
ADD: There's another answer and comments which seem to be based on knowledge of history and have some idea how 72 ppi happened to become selected, not for ex. 82 ppi.
