I took all measures - x-height, line spacing, and line length - in millimeters.
First, I will present the results of the x-height and line spacing measurements.
The mean x-height of continuous text in the three types of documents is identical: 1.
They measured the x-height in 100 American newspapers.
It also appears that there is a preference to make the x-height of continuous texts 1.
The strength of the relationship between x-height and line spacing is indicated by a correlation coefficient.
A second indication that typographic guidelines and practical documents differ is that the line spacing for type with the same x-height varies considerably.
Unfortunately, it is not possible to relate the actual measures of x-height and line spacing of text to the original point sizes in which a text was specified.
We can say with some confidence that a serifed type with a small or moderate x-height set to moderate line length could comfortably have line spacing equal to the type size ("set solid").
Types with large x-heights tend, as we have seen, to include white space within the characters, which may diminish the white track; types with smaller x-heights often create clearer white tracks.
Designs often varied between very small sizes, text sizes, and display sizes or titlings; x-heights in the smallest sizes might be relatively larger than in text sizes, and the body - the built-in spacing mechanism - of a titling might be relatively narrower than in the text sizes, so that displayed type did not "fall apart.