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Excerpts from netnews.comp.lang.postscript: 15-Jan-90 72.27! (was Re:
ruler.ps - .. Randal Schwartz@iwarp.in (915)

> OK, but if it *really* matters, it's 72.27 points per inch.  See
> Knuth's METAFONT page 33, for example, although I suppose a chart in
> Websters probably has the same information.


OK, here we go again. What is the size of a point. Well, webster defines it as:

    15) n, a unit of measurement: as
       a1) n, a unit of counting in the scoring of a game or contest
       a2) n, a unit used in evaluating the strength of a bridge hand
       b) n, a unit of academic credit
       c) n, a unit used in quoting prices of stocks, shares, and various
          commodities
       d) n, a unit of about {1/72} inch used to measure the belly-to-back
          dimension of printing type

    point_system
       n, a system in which printing type and spacing materials are made
       in sizes that are exact multiples of the point

It is obvious from these definitions, that, originally, there is no
clear definition. The one quoted by Knuth is probably from one of those
standard committees. My bet is that if you walk around various printing
shops with a ruler (a real one, not a silly thing printed out with
PostScript), ask the printers to set a line of 36 Pica (in case you want
to know, that is 432 points or about 6 inch)  and measure it, you will
find that most times it won't meet the official standard committee
specs. (It is also likely that it won't really match the size according
to the spec of the typesetter manufactor).

In practice, the size of a point varies in a arond 10% of 72 and inch,
and there are a lot of reasons why. 

So if it really matters, the size of a Pica point depends on the situation.

	jaap

Let me finish with an old article:

17-Oct-88 Re: Definition of Point David Slocombe@sq.uucp (2591)

In article <YXJVEOy00UkP40aWB1@andrew.cmu.edu>
jaap+@andrew.cmu.edu (Jaap Akkerhuis) writes:

>Bear in mind that all these nifty numbers hardly make any sense at all in the
>real world. I know of at least two models of a typesetter from the same
>manufacturer which have an apparently a different size for a point.
This is not
>a big deal, since it is marginal. However, it starts to become annoying when
>you want to combine output of both versions in one document. The linelength
>will vary enough to be noticable.

Too right, Jaap! A publishing house a block away from us has a
Linotron 101 laser phototypesetter which does not give you anything
like an exact pica in the direction of the film motion, because,
long ago, a serviceman repairing the machine went off without
checking that the *analog* adjustment that controls the scale in
that direction was reset correctly after he had repaired the machine.

It was a Friday, and an awful lot of work had to be done over the
weekend on all sorts of work-in-progress, so the staff "made do"
and worked out how to adjust what they were specifying so that the
actual physical measurements came out right. After that it was
awfully hard to find a time when it would be OK for the serviceman
to come back and readjust the machine, since you couldn't afford to
have corrections on jobs started that weekend not come out with
EXACTLY the same measures.... and the problem was renewing itself with
each new job started!

The upshot is that that Linotron 101 has been running with its
scale set wrong ever since.

Fortunately, in sqtroff there is a ".scale" request:

	.scale c numerator denominator [round | trunc]

where:
   "c" is a single letter that is the new troff scale unit (or an old
   one if it is being redefined).
   
   "numerator" is a non-negative integer (or number register) with
   some already-existing scale-indicator attached.
   
   "denominator" is a positive integer or register.
   
   "round" and "trunc" are keywords that set the way fractions will
   be handled with this new scale-indicator.

This new request was created to enable Europeans and fussy Americans
to redefine the point and Pica (the default is still 1/72 inch), but
you can imagine all the interesting uses it can be put to, including
dealing with permanently-misadjusted phototypesetters!

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SoftQuad Inc.				(800) 387-2777 (from U.S. only)
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