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In The Modification of Letter Forms Stanley Hess wrote:
"The horizontal line of some letters creates a lateral axis, causing these letters to appear inordinately wide. This is most apparent when there is not only a juncture at mid-point, but the letter contains horizontal elements as well. In order to restore stable proportions, such letters (B,E,F,P,R,S,a,e,s,2,3,5,6,8,9) need to be optically condensed."
Hess goes on to give other optical principles, including this one:
"Top units of letters with stacked shapes are contracted 1/20th the module in relation to lower units on either or both sides of the letter."
In another thread I referred to these optical principles--without citing the source--and also argued that the narrowing of stacked letters generally serves to maintain even color in words.
In arguing about how narrowing helps even color I was following ideas from David Kindersley, as well as testing things with my own eyes. According to Kindersley, the total advance width of a character is affected by the total amount of black in the letter. However, this total is a weighted sum, with the black at the left and right extremes counting for more than the black in the middle.
Kindersley's theory has complex implications, but it does mean that the dividing black strokes tend to push the ideal side bearings out wider. This need for wider side bearings may contradict the need for visually even spaces between letters. Thus in order to achieve both even color and even spacing, narrowing stacked letters helps. It reduces the need for wider side bearings, and hence brings the color and spacing into line with non-stacked letters.
Now this narrowing is not an absolute rule, as thinning some of the horizontal stokes will affect how wide you can go, and weight on the ends of strokes will also affect things. Thus for example the wide S (compared to Classical forms) in Bodoni and other "Modern" or "Romantic" (Bringhurst) faces, has thinner horizontal arches, and the thinness is maintained further toward the double curved diagonal stroke. "Moderno," currently the display type here, is an example.
In that thread Eben responded:
"I am sorry but I think the idea that an a (or s) has to be narrow to make it look even on the page is silly. It’s an optical question not a question of tradition or anything else.
"Carl’s point about this [typeface under discussion] being designed with display in mind is a more important point even than mine. A great text face with great even color wouldn’t work well in this context. It would be far too dull."
Eben, I didn't say that the narrowing of divided letters was "a question of tradition". I pointed out that traditionally admired letter designs *reflect* this optical principle pretty consistently. To me this lends weight to its being a correct principle. The fact that eg the higher contrast S goes together with a wider S in much-admired designs to me confirms that Hess and Kindersley are on to something real, even though they only have part of the story.
In other words, a designer is not at liberty to push individual letter forms every which way, and have the typeface not suffer in terms of optical balance of individual letters, and evenness in color in word forms. You can mess with one letter, but then what you do in one is going to have to be harmonized in all the other letters.
For this reason I think your implication that evenness of color is "dull" is a fundamental misunderstanding of good type. Type is still type, not lettering, and it needs to be able to set words at will, not find a design that suits one or two words, as lettering can do. Great display type--which is where I put Bodoni and Didot--can have even color, and is the better for it when it does.
Text type needs other qualities in addition to even color, qualities that make it work at small sizes. But great display type also needs even color, in my opinion.