Editing PostScript (section)

=== Units of length ===
PostScript uses the [[Point (typography)|point]] as its unit of length. However, unlike some of the other versions of the point, PostScript uses exactly 72 points to the inch. Thus:

: {{math|1=1 point = {{sfrac|1|72}} inch = {{sfrac|25.4|72}} mm = {{sfrac|127|360}} mm = 352.777…  micrometers}}

For example, in order to draw a vertical line of 4&nbsp;cm length, it is sufficient to type:

<syntaxhighlight lang="postscript">
0 0 moveto 
0 113.385827 rlineto stroke
</syntaxhighlight>

More readably and idiomatically, one might use the following equivalent, which demonstrates a simple procedure definition and the use of the mathematical operators <code>mul</code> and <code>div</code>:
<syntaxhighlight lang="postscript">
/cm {72 mul 2.54 div} def               % 1 inch = 2.54 cm exactly
 0 0 moveto
 0 4 cm rlineto stroke
</syntaxhighlight>

(Technically, most printers have a construction-implied unprintable margin around the physical borders of the sheet, and the 0 0 coordinates are calibrated to its corner,<ref name=PLR/>{{rp |at=section 4.3.1}} so you might have to use a different starting point to actually see something.)

Most implementations of PostScript use [[single-precision]] reals<ref name=PLR/>{{rp |at=appendix B}} (24-bit mantissa), so it is not meaningful to use more than 9 decimal digits to specify a real number, and performing calculations may produce unacceptable round-off errors.