This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
Choosing a Mac monitor used to be simple. Like the Model-T Ford you
could have any color you wanted as long as it was black (and white),
9", 512 by 342 pixels, 72 dpi. The PC drones made fun of the small
size and lack of color, but it was obvious to any unprejudiced person
who looked at a Macintosh that its display was far superior to the
CGA and EGA monitors being foisted on ignorant PC consumers. Mac
monitors are no longer so simple. Now one needs to be concerned with
such arcana as resolution, size, bit depth, dot pitch, and refresh
Size is the most obvious characteristic of a monitor. It's measured
diagonally from one corner of the screen to the opposite corner.
Actual monitor area is roughly proportional to the square of the
diagonal length so a twenty-inch monitor is more than four times as
large as a nine-inch monitor. Most manufacturers cheat on their
monitor sizes by measuring from one corner of the screen (or even the
case) to the other rather than from one edge of the visible display
to the other. Then they round up to the nearest inch with the result
that most "fourteen-inch monitors" are closer to twelve and a half
inches when measured truthfully. For many years Apple was one of the
most honest manufacturers, advertising it's twelve and a half inch
monitor as a thirteen inch monitor while other manufacturers touted
their "larger fourteen-inch" twelve and a half inch monitors.
However Apple has succumbed to the pressures of the market, and like
everyone else it now advertises twelve and a half inch monitors as
"fourteen inch displays."
Of course it's not the size that matters; it's how you use it.
Resolution defines how much information can be squeezed onto the
screen. Most monitors sold today are "multi-sync"; that is they are
capable of displaying more than one resolution. A fifteen inch
monitor at 1024 by 768 pixels displays two and a half times as much
information as the same monitor at 640 by 480 pixels. However
everything will appear smaller at the larger resolution since the
monitor has to fit more pixels into the same space. The clearest
resolution for a monitor is whatever comes closest to fitting 72
pixels (or dots) into each inch. This is the dpi rating of the
monitor. 72 dpi is the proper "WYSIWIG" (Pronounced Whizzy-wig, What
you see is what you get) resolution though some people prefer to
work at a higher resolution that fits more information on the screen.
Here are the WYSIWIG resolutions for common monitor sizes. If you do
the math you'll notice that the resolutions seem too small for the
given size. That's because I've listed sizes here in their commonly
advertised form rather than by the actual paintable area on the
Size WYSIWIG Resolution 9 512 by 342 12 512 by 384 13,14,15 640 by 480 16,17 832 by 624 20 1024 by 768 21 1152 by 870 25 1280 by 1024