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?