Citations and references

Reference databases

For bibliographic citations, while you can include reference sources directly in your document, usually you will get that information from one or more external files. Such a file is a database of references, containing the information in a processing-friendly format. Using one or more reference databases lets you re-use information and avoid manual formatting.

Reference databases are normally referred to as ‘BibTeX files’ and have the extension .bib. They contain one or more entries, one for each reference, and within each entry there are a series of fields. Let us look at an example.

@article{Thomas2008,
author  = {Thomas, Christine M. and Liu, Tianbiao and Hall, Michael B.
and Darensbourg, Marcetta Y.},
title   = {Series of Mixed Valent {Fe(II)Fe(I)} Complexes That Model the
{H(OX)} State of [{FeFe}]Hydrogenase: Redox Properties,
Density-Functional Theory Investigation, and Reactivity with
Extrinsic {CO}},
journal = {Inorg. Chem.},
year    = {2008},
volume  = {47},
number  = {15},
pages   = {7009-7024},
doi     = {10.1021/ic800654a},
}
@book{Graham1995,
author    = {Ronald L. Graham and Donald E. Knuth and Oren Patashnik},
title     = {Concrete Mathematics},
year      = {1995},
}


This is an entry for an article and another for a book; these are by far the most common types. Each database entry type starts with @, as shown, and all of the information then sits within a brace pair.

The various fields we need are given in key-value format, apart from what is known as the ‘key’: the ‘name’ of the citation. You can use whatever you like, as it’s just a label, but above we’ve chosen to use the name of an author plus the year: this is a common approach.

Exactly which fields you need to give depends on the type of entry, but most of these are quite obvious. You might notice that in the author field, each entry is separated by and. This is essential: the format of the output needs to know which author is which. You might also notice that in the article title, some entries are in an extra set of braces; these are there to prevent any case-changing being applied.

Editing .bib files by hand is rather tedious, so most people use a dedicated editor. JabRef is widely used and cross-platform, but there are several other interfaces available. If the reference contains a DOI (Digital Object Identifier), you may want to try doi2bib to easily get the BibTeX entry. But make sure to check if the entry is correct!

Here, we will use the short example database above for our demonstrations: we have ‘saved’ it as learnlatex.bib.

Transferring information from the database

To get the information into your document there are three steps. First, use LaTeX to compile your document, which creates a file with a list of the references that your document cites. Second, run a program that takes information from the database of references, picks out the ones that you use, and puts them in order. Finally, compile your document again so that LaTeX can use that information to resolve your citations. Usually it will require at least two compilations to resolve all the references.

For the second step, there are two systems in wide use: BibTeX and Biber. Biber is only ever used with a LaTeX package called biblatex, whereas BibTeX is used with either no packages at all or with natbib.

Running a second tool as well as LaTeX is handled in different ways by different editors. For our online examples, there are some ‘behind the scenes’ scripts that do everything in one go; that’s true for LaTeX.Online, LaTeX-on-HTTP and Overleaf. Your editor might have a single ‘do stuff’ button or you might have to choose to run BibTeX or Biber manually between LaTeX runs.

The format of citations and references is independent of your BibTeX database, and is set by what is known as a ‘style’. We will see that these work slightly differently in natbib and biblatex, but the general idea remains: we can choose how citations appear.

The natbib workflow

Whilst it is possible to insert citations into a LaTeX document without any packages loaded, this is rather limited. Instead, we will use the natbib package, which allows us to create different types of citation and has a lot of styles available.

The basic structure of our input is as shown in this example.

\documentclass{article}
\usepackage{natbib}

\begin{document}
The mathematics showcase is from \citet{Graham1995}, whereas
there is some chemistry in \citet{Thomas2008}.

Some parenthetical citations: \citep{Graham1995}
and then \citep[p.~56]{Thomas2008}.

\citep[See][pp.~45--48]{Graham1995}

Together \citep{Graham1995,Thomas2008}

\bibliographystyle{plainnat}
\bibliography{learnlatex}
\end{document}


You can see that we can cite different entries in the database by giving their key. The natbib package offers both textual and parenthetical citation styles, \citet and \citep, respectively. The reference style is selected by the \bibliographystyle line; here we’ve used the plainnat style. The bibliography is actually inserted by the \bibliography line, which also picks the database(s) to use; this is a comma-separated list of names.

Page references can be added to the citation with an optional argument. If two optional arguments are given, the first goes in front of the citation label for a short note and the second after the label for a page reference.

The setup above uses author-year style, but we can make use of numeric citations. That is done by adding the numbers option to the natbib line.

The biblatex workflow

The biblatex package works slightly differently to natbib, as we select the databases in the preamble but print it in the document body. There are some new commands for this.

\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\addbibresource{learnlatex.bib} % file of reference info

\begin{document}
The mathematics showcase is from \autocite{Graham1995}.

Some more complex citations: \parencite{Graham1995} or
\textcite{Thomas2008} or possibly \citetitle{Graham1995}.

\autocite[56]{Thomas2008}

\autocite[See][45-48]{Graham1995}

Together \autocite{Thomas2008,Graham1995}

\printbibliography
\end{document}


Notice that \addbibresource requires the full database filename, whereas we omitted the .bib for \bibliography with natbib. Also notice that biblatex uses rather longer names for its citation commands, but these are all quite easy to guess.

Again, short text before and after the citation can be inserted with the optional arguments. Note that the page numbers need not be prefixed with p.~ or pp.~ here, biblatex can automatically add the appropriate prefix.

In biblatex, the reference style is picked when we load the package. Here, we’ve used authoryear, but there is a numeric style and many others are also available.

Exercises

Try out both the natbib and biblatex examples. For natbib, you’ll need to run LaTeX, BibTeX, LaTeX, LaTeX; for biblatex, it’s LaTeX, Biber, LaTeX. Find out how to do that in your editor, or try the Overleaf and LaTeX Online automation.

See what happens when you create new database entries and new citations. Add a citation that’s not in the database and see how it appears. Experiment with the numeric option to both packages.

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