# [Tex/LaTex] Why is the linespread factor as it is

line-spacing

The blog post Quick note on line spacing discusses line spacing and it says

To change the line spacing for the entire document, you can use the
linespread command in your preamble:

\linespread{<factor>}


The factor is somewhat confusing. For double-spacing you have to use
1.6 and for one-and-a-half spacing 1.3. Not very intuitive, but I'm
sure there is a reason for it.

This unintuitive factor triggered my curiosity. What is the reason for it being as it is?

This question is just motivated by my curiosity and I have no practical reason for asking it. Feel free to close it if it's inappropriate or see at as post in the same vein as \nothing, \varnothing and \emptyset in that it asks about history or some design decision.

With the LaTeX standard classes (article, book, and report) and no class options added, \normalsize results in a font size (size of the largest glyphs in a font -- typically, braces) of 10pt and a \baselineskip (vertical skip between the base lines of two successive lines of type) of 12pt. The ratio between font size and \baselineskip is 1.2.
The linespread command (which must be issued in the document preamble) may be used to change the \baselineskip without changing the font size.
A possible definition of \onehalfspacing and \doublespacing is that the ratio between font size and \baselineskip should be 1.5 resp. 2. Because the "basic" ratio for 10pt is 1.2, a multiplier of 1.25 and (approximately) 1.667 has to be applied -- and this is basically what the setspace package does. ("Basically" because it retains the ratio of 1.2 for footnotes and the like.)