her DVI output file will represent the design document regardless of
magnification.

Caveat:  Due to the manner in which the current implementation of TEX writes
Press files, it is not advisable to change the value of parameter 12 in the
middle of a TEX run.  If you want to produce magnified output, you should
reset parameter 12 once very early in your document by using the  \chpar12
control sequence, and from then on leave it alone.  Another Caveat below
discusses the situation in more detail.

* Magnified fonts, an extension to \font:
The magnification mechanism has been extended to include font specifications
as well:  in order to print a document that is photographically magnified, it
is essential to use magnified fonts.  A font is specified by the "\font"
control sequence, which now has the syntax
	"\font <fontcode>=<filename> at <dimen>".
The "at" clause is optional.  If present, the dimension specified is taken
as the desired size of the font, with the assumption that the font should be
photographically expanded or shrunk as necessary to scale it to that size.
For example, the two fonts requested by the control sequences 
	"\font a=CMR10 at 5pt" and "\font b=CMR5 at 5pt"
will look somewhat different.  Font a will be CMR10 photographically reduced
by a factor of two, while font b will be CMR5 at its normal size (so it should
be easier to read, assuming that it has been designed well).

The dimension in a font specification can use any units, either standard or
"true".  The interpretation of "true" here is identical to its interpretation
in the specification of any other distance:  asking for a font "at 5pt"
requests that the font be 5 points in size in TEX's "design document", while
asking for a font "at 5truept" requests that the font be 5 points in size
after the scaling implied by the "magnification" factor.

If the "at <dimen>" clause is omitted, TEX defaults the requested size to the
design size of the font, interpreted as a design (non-"true") distance. 
Thus, the control sequence "\font a=CMR10" is equivalent to the sequence
"\font a=CMR10 at 10pt", assuming that the designer of cmr10 has indeed
told TEX that cmr10 is a 10-point font.

Caveat:  This extension allows the TEX user to request any magnification of
any font.  In general, only certain standard magnifications of fonts will be
available at most printers.  The user of TEX at any particular site must be
careful to request only those fonts that the printer can handle. 

Caveat:  As mentioned above, you shouldn't change the value of parameter 12
in the middle of a run.  TEX uses the value of parameter 12 in the following
three ways:  (i) whenever the scanner sees a "true" distance, it divides
by the current magnification;  (ii) at the end of every page, the output
module scales all distances by the current magnification while writing this
page of the Press output file;  and (iii) at the very end of the TEX run,
the output module uses the current magnification to scale the requested sizes
of all fonts.  Given this state of affairs, it is best not to change
parameter 12 once any "true" distance has been scanned and once any page
has been output.

* Page 40,  there are two new units of distance:
	bp	"big point"  (one inch equals exactly 72 "big points")
	mi	mica  (one millimeter equals 100 micas)