# [TUHS] Reconstructed and newly set UNIX Manual

Nelson H. F. Beebe beebe at math.utah.edu
Sun Oct 28 00:18:06 AEST 2018

```Angelo Papenhoff <aap at papnet.eu> writes about the conversion of
printer points to other units:

>> >From my experience in the world of prepress 723pts == 10in.
>>
>> Then Adobe unleashed PostScript on us and redefined the point
>> so that 72pt == 1in.
>>
>> Ibunaware of any other definitions of a point.

The most important other one is that used by the TeX typesetting
system: 72.27pt is one inch.  TeX calls the Adobe PostScript one a big
point: 72bp == 1in.  Here is what Don Knuth, TeX's author, wrote on
page 58 of The TeXbook (Addison-Wesley, 1986, ISBN 0-201-13447-0):

>> ...
>>     The units have been defined here so that precise conversion to sp
>>     is efficient on a wide variety of machines. In order to achieve
>>     this, TeX's ``pt'' has been made slightly larger than the official
>>     printer's point, which was defined to equal exactly .013837in by
>>     the American Typefounders Association in 1886 [cf. National Bureau
>>     of Standards Circular 570 (1956)]. In fact, one classical point is
>>     exactly .99999999pt, so the ``error'' is essentially one part in
>>     10^8.  This is more than two orders of magnitude less than the
>>     amount by which the inch itself changed during 1959, when it
>>     shrank to 2.54cm from its former value of (1/0.3937)cm; so there
>>     is no point in worrying about the difference. The new definition
>>     72.27pt=1in is not only better for calculation, it is also easier
>>     to remember.
>> ...

Here sp is a scaled point: 65536sp = 1pt.  The distance 1sp is smaller
than the wavelength of visible light, and is thus not visible to
humans.

TeX represents physical dimensions as integer numbers of scaled
points, or equivalently, fixed-point numbers in points, with a 16-bit
fraction.  With a 32-bit word size, that leaves 16 bits for the
integer part, of which the high-order bit is a sign, and the adjacent
bit is an overflow indicator.  That makes TeX's maximum dimension on
such machines 1sp below 2^14 (= 16,384) points, or about 5.75 meters
or 18.89 feet.

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