[Tex/LaTex] \begin{doublespacing} Versus \begin{spacing}{2.0}

Question

The following \begin{doublespacing} only affects FIRST.

\documentclass{article}
\usepackage{setspace}
\begin{document}
\begin{doublespacing}
FIRST

SECOND

\end{doublespacing}
THIRD

FOURTH
\end{document}

In contrast, \begin{spacing}{2.0} affects both FIRST and SECOND.

\documentclass{article}
\usepackage{setspace}
\begin{document}
\begin{spacing}{2.0}
FIRST

SECOND

\end{spacing}
THIRD

FOURTH
\end{document}

Why are they different?

Answer 1

With a 10pt (default) font size, the setspace package sets the following stretch factors for \onehalfspacing and \doublespacing: 1.25 and 1.667. I guess the rationale for this is that for many common fonts -- including Computer Modern and Times Roman -- the baselinestretch is 1.2 times the nominal font size. Sure enough, 1.25 * 1.2=1.5 -- 50% more than the nominal font size -- and 1.667 * 1.2=2.0 = twice the nominal font size.

If, in contrast, you run \begin{spacing}{2.0} or, equivalently, \setstretch{2.0}, you get even wider spacing -- by a factor of 1.2 -- than "doublespacing".

---

Addendum to address the OP's follow-up comment: I'm not sure I understand the comment completely, but I believe it asserts that using the spacing environment (provided by the setspace package) with a factor of 1.667 produces line spacing that's different from using the command \setstretch{1.667}.

Please consider the following test program, which twice typesets a paragraph of filler text (produced via \lipsum[3]) in a two-column document. The left-hand column uses the environment \begin{spacing}{1.667} ... \end{spacing}, whereas the right-hand column uses the command \setstretch{1.667}. Sure enough, the line spacing in both paragraphs is the same.

The outcome is the same if one (a) replaces \begin{spacing}{1.667} with \doublespacing and (b) omits the instruction \end{spacing}.

\documentclass{article} 
\usepackage{setspace,multicol,lipsum} 
\begin{document} 
\begin{multicols}{2} 

% Left-hand column:
\begin{spacing}{1.667} 
\lipsum[3] 
\end{spacing} 

% Right-hand column:
\setstretch{1.667} 
\lipsum[3] 

\end{multicols} 
\end{document}