Nominal values for \parindent and other paragraph metrics

Question

In memoir, when I run

\documentclass{memoir}
\begin{document}
\showthe\parindent
\end{document}

I get:

17.62482 pt

My questions are :

1. Why that value? This number seems to capture something with high precision. Was it adjusted manually? If not, how do memoir or LaTeX compute it?
2. Is there a reference on what are "good-looking" values for parameters like parindent, parskip, etc? What values are typically used/recommended?

Answer 1

The memoir class sets the \parindent to 15pt in one-column mode at 10pt size, to 17pt at 11pt size, to 1.5em (based on the default Computer Modern font) at 12pt size. In two-column mode the \parindent is always set to 1em (again based on the default Computer Modern font).

Usually 1em corresponds approximately to the font design size, so you get (slightly less than) 18pt in this case. Computer Modern em is a bit less than the design size at 12pt (Knuth's decision), but it's exactly 10pt for cmr10.

It's a designer's decision. For example, the parindent is smaller in two-column mode, to compensate for the reduced column width.

If you want to change it based on the actual font used in the document, the right place to do it is just after \begin{document}, unless you prefer an absolute dimension. But 1.5em or 1em can be considered standard.

Answer 2

The usual value of \parindent is 1em (as used by memoir in some cases), with the intention of creating (considering line distances) a "visual square" of empty space.