Optimal Characters per line for readers

From the ‘Memoir Book Class” Manual:

Experiments have shown that the number of characters in a line of single column text
on a page should be in the range 60 to 70 for ease of reading. The range may be as much
as 45 to 75 characters but 66 characters is often considered to be the ideal number.
shorter and the eye is dashing back and forth between each line. Much longer it is hard to
pick up the start of the next line if the eye has to jump back too far — the same line may
be read twice or the following line may be inadvertently jumped over. For double column
text the ideal number of characters is around 45, give or take 5 or so.
Bringhurst [Bri92] gives a method for determining the number of characters in a line
for any font: measure the length of the lowercase alphabet and use a copyfitting table that
shows for a given alphabet length and line length, the average number of characters in
that line. Table 2.2 is an abridged version of Bringhurt’s copyfitting table. For example, it
suggests that a font with a length of 130pt should be set on a measure of about 26pc for a
single column or in an 18pc wide column if there are multiple columns.
The vertical height of the typeblock should be constant from page to page. The lines
of text on facing pages should be aligned horizontally across the spine, which also means
that they will be at the same place on both sides of a leaf. Alignment across the spine
means that the eye is not distracted by an irregularity at the centre of a spread, and leaf
alignment stops ghosting of text through a thin page, giving a crisper look to the work. So,
the spacing between lines should be constant. This implies that the depth of the typeblock
should be an integral multiple of the space required for each line; that is, be specified as
a multiple of the leading. A ten point type, for example, will normally have two points
between lines, to give a leading of 12 points. This can be written as 10/12. Usefully, one
pica is 12 points so with a 12pt leading vertical distances can be conveniently expressed in
picas (one pica per line). Another implication of this is that any space left for illustrations
or tables, or the amount of space taken by chapter and section headings should also be an
integer multiple of the leading.


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