[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

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.

- Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
- University of Utah                    FAX: +1 801 581 4148                  -
- Department of Mathematics, 110 LCB    Internet e-mail: beebe at math.utah.edu  -
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