When I run the following code:
\documentclass[9pt]{article}
\usepackage{setspace}
\linespread{2}
\def\lipsum{Lorem ipsum dolor sit amet, consectetuer adipiscing
elit. Ut purus elit, vestibulum ut, placerat ac, adipiscing
vitae, felis. Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a}
\begin{document}
{\Large
\lipsum
\lipsum
}
{\Large
\lipsum
\lipsum
}
\end{document}
The result is:
As you can see, the linespacing of the second paragraph is different. What's wrong? I want to understand the reason so that I won't make a mistake in a similar context.

\par) before switching back to another font. Otherwise the line spacing will be taken from the font active at the end of the paragraph. – samcarter_is_at_topanswers.xyz Mar 28 '24 at 21:24articledocument class does not recognize9ptas a valid option -- and thus simply ignores it. – Mico Mar 28 '24 at 21:26