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As discussions in An Ideal Page: Line length, Leading, Margins, Page Size, Etc. and Cyrillic (Russian) ideal number of characters per line go, the topics discussed make absolutely no sense to me. Discussing the “optimal number of characters on line” begs the question — it presumes that when one increases font size by a certain percentage, the optimal line length should increase by the same percentage. And I find this presumption silly.
But thinking about this more, the may be some other power law governing the relation between the font size and line length. Suppose one is required to prepare two copies of a book, one set in 10pt, another in 14pt; suppose that the choice of page size and layout is completely in your hands. It is natural to assume that the larger-font variant will be designed to have somewhat larger page size. Probably not by 40%, but definitely larger.
My question is: how much larger? The increase in percents will determine the exponent of the power law: if the “desired” increase in size is 18%, it is ½-power; if 12%, ⅓-power etc. (Was this question discussed before? I can’t find any reference….)
I see one major complication: choosing a page size should depend on the expected position of the book when read. People reading in chaise-long-position would react on a heavier book differently than people who prefer reading at a desk. Is there any wisdom in addressing such differences?
BTW, why I consider the rules like “1½ lowercase alphabet” ridiculous: a couple of thought experiments. In XVIII century reform of Russian alphabet, 6 of 46 “duplicate” letters were removed. Does it mean that the optimal length of line should have been decreased 15%? In XX century reform of Russian alphabet, 3 of 36 excess letters were removed. Does it mean that the optimal length of line should have been decreased 10%?
To add insult to injury, note that difference between these two reforms: the XX century reform removed the terminal hard sign Ъ. This reduced the average length of a word (about 10%?). And this may have a chance of affecting the optimal line length (but here I consider line length in the case of a prescribed-in-advance font size!).
To summarize: I think that discussion of optimal line length makes sense when one knows in advance the font size, and the expected reading pose. It does not make sense without this knowledge.