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Mark Levinson reports: <<I use even sizes, odd sizes, half sizes, but I do try
to follow the rule (and maybe this is the general rule of which "even-only" is
a variant) that different sizes in the same type family should differ by at
least 2 points. For example, if you're using Garamond 12 and you want something
a bit smaller, you should skip Garamond 11 and go down to Garamond 10. The
rationale, I believe, is that the smaller differences are irritating because
the reader's eye is unsure whether a difference exists or not.>>
That fits with both personal experience and what I've read, but I'd generalize
the rule more than Mark has done: the difference should typically be at least
20% (probably the source of the 10 points + 20% = 12 points that Mark used as
an example). This will depend on the font and size to some extent, but it's
still a reasonably robust rule of thumb. It's also got some correspondences
with George Miller's old research on perception that led to the "rule of 7 plus
or minus 2". In many scales, humans can only easily distinguish between 7 or so
discrete steps at a time, and if you opt for a larger margin of safety (7-2 =
5) you get 5 steps. On a percentage scale, that would be 100/5=20%.
This may simply be me committing psychobabble, or it may be a legitimate
extension of Miller's research. But I've found in other areas (e.g., picking
shades of grey for use in labeling graphs), a difference of at least 20% is
required for the difference to be easily detectable.
--Geoff Hart ghart -at- netcom -dot- ca
Pointe-Claire, Quebec, Canada
"Most business books are written by consultants and professors who haven't
spent much time in a cubicle. That's like writing a firsthand account of the
Donner party based on the fact that you've eaten beef jerky."--Scott Adams, The