From Wikipedia: Dots per inch - Wikipedia, the free encyclopedia Since the 1980s, the Microsoft Windows operating system has set the default display "DPI" to 96 PPI, while Apple/Macintosh computers have used a default of 72 PPI. [3] These default specifications arose out of the problems rendering standard fonts in the early display systems of the 1980s, including the IBM-based CGA, EGA, VGA and 8514 displays as well as the Macintosh displays featured in the 128K computer and its successors. The choice of 72 PPI by Macintosh for their displays arose from the convenient fact that the official 72 points per inch mirrored the 72 pixels per inch that appeared on their display screens. (Points are a physical unit of measure in typography, dating from the days of printing presses, where 1 point by the modern definition is 1/72 of the international inch (25.4 mm), which therefore makes 1 point approximately 0.0139 in or 352.8 µm). Thus, the 72 pixels per inch seen on the display had exactly the same physical dimensions as the 72 points per inch later seen on a printout, with 1 pt in printed text equal to 1 px on the display screen. and an other one: Point (typography) - Wikipedia, the free encyclopedia Desktop publishing point The desktop publishing point (DTP point) or PostScript point is defined as 1 ⁄ 72 or 0.0138 of the international inch, making it equivalent to 0.3527 mm. Twelve points make up a pica, and six picas make an inch. This specification was developed by John Warnock and Charles Geschke when they created Adobe PostScript . It was adopted by Apple Computer as the standard for the display resolution of the original Macintosh desktop computer and the print resolution for the LaserWriter printer. ...and that's the reason why 72 is the magic number and used by InDesign.
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