bug#6930: 24.0.50; (elisp) Layout Parameters: What is ""the standard fringe width"?

[Drew Adams]

The text says this:
 
     The default width of the left and right fringes of windows in this
     frame (*note Fringes::).  If either of these is zero, that
     effectively removes the corresponding fringe.  A value of `nil'
     stands for the standard fringe width, which is the width needed to
     display the fringe bitmaps.
 
If Lisp code wants to know how wide the left fringe is for a certain
frame, it can look at the value of parameter `left-fringe' (and at
variable `fringe-mode').  But if parameter `left-fringe' is nil, then
the fringe width is what?  It says the width is "the standard fringe
width".
 
Well, what's that?  "The width needed to display the fringe bitmaps."
Huh?  And how does it find out how wide that is for the target frame?
 
In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2010-08-16 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4) --no-opt --cflags
-Ic:/imagesupport/include'