What is best method to insert an ellipsis in a (Xe)TeX document?

Question

What is best method to insert an ellipsis in a (Xe)TeX document?

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Times New Roman}

\begin{document}
a...b a ... b %or

a\ldots b a \ldots  b %or

a…b a … b  

$a...b  a ... b$ %or

$a\ldots b  a \ldots  b $ %or

$a…b  a … b  $
\end{document} 

Their results seem to be similar.

Answer 1

You asked:

Is there any advantage in using \ldots instead of ...?

Yes! If you type ... you'll get some rather-closely spaced dots. In contrast, with \ldots the dots are correctly spaced for a typographic ellipsis.

The following MWE illustrates some of the visual differences created by ... and \ldots. The differences are readily apparent in both text mode and math mode.

\documentclass{article}
\begin{document}
yes and no ... yes and no \ldots\ yes and no ...

$\{x_1,x_2,...,x_n\}$ vs.\ $\{x_1,x_2,\ldots,x_n\}$
\end{document}

Naturally, if your language is not English -- for which \ldots was designed initially -- you'll want to make sure that the ellipis created by \ldots conforms to your language's typographic conventions. @egreg's comment suggests that this may well be a concern for the French case. For such situations, you may want to use the commands provided by a specialized package such as ellipsis or csquotes rather than the "standard" \ldots command.

Addendum prompted by a follow-up question by @moose: In LaTeX, there's also the command \dots. This raises the natural question, what is the difference between \ldots and \dots? The simple answer is: There's no difference in LaTeX. (I'm not sure if this applies to Plain TeX as well; I don't have the TeXbook at hand today...) The definitions of these two commands (from latex.ltx) are as follows:

 \DeclareRobustCommand{\dots}{%
    \ifmmode\mathellipsis\else\textellipsis\fi}
 \let\ldots\dots

The \textellipsis instruction, in turn, is defined in the LaTeX kernel as follows:

\DeclareTextCommandDefault{\textellipsis}{%
    .\kern\fontdimen3\font
    .\kern\fontdimen3\font
    .\kern\fontdimen3\font}

Answer 2

I suggest using \ldots in combination with the package ellipsis, because that gives you better control over spacing than .... Using \ldots alone (without ellipsis) is better than ... because it gives you more flexibility. For example, you may redefine the spacing by redefining \ldots. On the other hand, ellipsis makes it easy to fine tune spacing, and takes care of several other spacing issues, for example ellipsis in combination with another full stop.

In case you want to fine tune ellipsis, the distance between the dots is change simply by the command:

\renewcommand{\ellipsisgap}{0.1em} %  or another value

Also, by adding the option [xspace], i.e.

\usepackage[xspace]{ellipsis}

terminating the \ldots command by \ is not necessary (but even xspace makes errors, as documented in the manual). If you use ellipsis, remember to load it after hyperref.

The package csquotes has the commands

\textelp{}
\textelp{<text>}
\textelp*{<text>}

that fill similar functions as ellipsis, and a lot of possibilities to automatically handle exception from the rules. For the same reasons, this is a better solution than ....

I recommend using \ldots with ellipsis (or substitutes), which gives better control over spacing, more flexibility and automatic typographical consistency in more situations.

Answer 3

You can achieve advantages of both (readability and good appearance) using Unicode Character 'HORIZONTAL ELLIPSIS' (U+2026)

If you have installed AutoHotKey use following script to convert ... to … automatically:

::...::…

In Windows, it can be inserted with Alt+0133.

In MacOS, it can be inserted with ⌥ Opt+; (on an English language keyboard).

In Linux, it can be inserted with AltGr+.