I have emacs 21.3.1 running on XP SP3.  When I evaluate the expression
using M-: (Meta-colon),
 
    (frame-width)
 
it returns 72.  However, if I look at the number of columns I can
display before the line wraps, it's actually 80.  Isn't frame-width
supposed to be the same as the number of columns that can fit on one
line?  (I'm using a fixed-width font, specifically
 
  (set-default-font
"-raster-Terminal-bold-r-normal-normal-12-90-96-96-c-*-ms-oemlatin")
 
Likewise, if I set the frame width with
 
    (set-frame-width (selected-frame) 72)
 
it sets up the window so that 80 columns fit on one line.  What gives?
Or is there another function I should use to obtain the number of
columns that fit on one line, and likewise, another function to set how
many fit on one line?