Only one \baselineskip per paragraph

The \baselineskip, which determines the space between lines, is not (as one might hope) a property of a line, but of a paragraph. As a result, in a 10pt (nominal) document (with a default \baselineskip of 12pt), a single character with a larger size, as:

{\Huge A}

will be squashed into the paragraph: TeX will make sure it doesn’t scrape up against the line above, but won’t give it “room to breathe”, as it does the text at standard size; that is, its size (24.88pt) is taken account of, but its \baselineskip (30pt) isn’t. This problem may be solved by a strut: the name comes from movable metal typography, and refers to a spacer that held the boxes (that contained the metal character shapes) apart. Every time you change font size, LaTeX redefines the command \strut to provide the equivalent of a metal-type strut for the size chosen. So for the example above, we would type

Paragraph text ...
   {\Huge A\strut}
   ... paragraph continues ...

This technique only works for such very short intrusions; if you need several lines, you should convert your intrusion into a quote environment, since it’s not possible to provide a \strut command for every line of the intrusion, in a sensible way, so proceed by:

\begin{quote}
  \Huge A LENGTHY TEXT ...
  SHOUTING AT THE READER!
\end{quote}

The contrary case:

Paragraph text ...
{\footnotesize Extended interjection ...
   ... into the paragraph.}
      ... paragraph continues ...

will look wrong, since the 8pt interjection will end up set on the 12pt \baselineskip of the paragraph, rather than its preferred 8.5pt. A \strut here is no help: there is no such thing as a “negative strut”, that draws lines together, so once more, one falls back on the quote to separate the interjection:

Paragraph text ...
\begin{quote}
  \footnotesize Extended interjection ...
  ... into the paragraph.
\end{quote}
... paragraph continues ...

The same effect is at work when we have something like:

Paragraph text ...
  ... paragraph body ends.
{\footnotesize Comment on the paragraph.}

Next paragraph starts...

which will set the body of the first paragraph on the constricted \baselineskip of the \footnotesize comment. Solve this problem by ending the initial paragraph before starting the comment:

Paragraph text ...
  ... paragraph body ends.
\par\nothtml{\noindent}
{\footnotesize Comment on the paragraph.}

Next paragraph starts...

(We suggest \noindent to make the comment look as if it is part of the paragraph it discusses; omit \noindent if that is inappropriate.)

A variation of the previous issue arises from a paragraph whose size is different from those around it:

{\Large (Extended) IMPORTANT DETAILS ...}

Main body of text...

Again, the problem is solved by ending the paragraph in the same group as the text with a different size:

{\Large (Extended) IMPORTANT DETAILS ...\par}

Main body of text...

FAQ ID: Q-baselinepar