[Tex/LaTex] Argument in “thebibliography”

Question

I was reading the explanation about thebibliography and was totally confused:

The thebibliography environment has an argument which should be a string at least the length of the widest entry label in the source list. Instead of using the numbers generated by the environment as entry labels, you can specify your own by using an optional argument to \bibitem, but remember to increase the length of the argument to the thebibliography command.

What does "widest entry label" mean? Practically, what should I put as the argument in

\begin{thebibliography}{?}

or is the argument not necessary?

Answer 1

The argument to \begin{thebibliography} is used for setting the hanging indentation of the bibliography items.

If you use numeric labels, then the argument should be a single digit (9 is commonly used) if there are less than ten items; two digits (commonly 99) if there are from 10 to 99 items and so on.

If you use symbolic labels (say Xyz12 or XY13) like BibTeX creates with the abbrv style, then you should pass the widest label, for instance

 \begin{thebibliography}{Xyz12}

or whatever abbreviation is the widest.

The correct argument is needed in order to make the labels flush left and the bibliography items to have precise hanging indentation.

Note that BibTeX is able to figure out the widest label when writing the thebibliography environment to the .bbl file.