If you look at a printer, it's usually rated at either 300 DPI, 600 DPI 
or 1200 DPI. So how many DPI do you get for a typical monitor?

To be completely precise, let's look at *pixels* per inch, since this is 
unambiguous.

Now, if I understand this correctly, a 14 inch monitor is actually 14 
inches across the diagonal. That means that the 14 inches is the length 
of the hypotenuse of a right-angle triangle. Assuming the monitor is 4:3 
aspect, we have a 3,4,5 triangle. Based on this, we have:

14" 4:3 = 11.2" x  8.4"
21" 4:3 = 16.8" x 12.6"

Suppose our hypothetical monitor can display 1280x768 pixels. Then in 
the horizontal direction, we have:

   1280 pixels / 11.2 inches = 114.3  pixels/inch
   1280 pixels / 16.8 inches =  76.19 pixels/inch

That's quite some variation. Hmm, let's try my actual monitor at home. 
Its resultion is the obscure figure 1680x1050, and it has the weird 
aspect ratio 16:10.

Apparently a triangle with sides of 16 and 10 has a hypotenuse who's 
length is equal to twice the square root of 89 - or approximately 18.87. 
That gives me screen dimensions of about 17.81" by 11.3". And so, we have:

   1680 pixels / 17.81 inches = 94.329 pixels/inch

In conclusion, it appears that most if not all current computer monitors 
have a density slightly above or below 100 pixels/inch. That's about 
tree times lower than the crappiest printer.

Now, admittedly, for colour images computer screens have the advantage 
of not requiring the halftoning that printers generally require. But for 
black text on a white background, printers win by a mile. (Even 
including the fact that computer screens can do antialias, which is 
generally a waste of time in printed text.)

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