\bibitem
........
`\bibitem[label]{cite_key}'
The `\bibitem' command generates an entry labelled by `label'. If
the `label' argument is missing, a number is generated as the `label',
using the `enumi' counter. The `cite_key' is any sequence of letters,
numbers, and punctuation symbols not containing a comma. This command
writes an entry on the `.aux' file containing `cite_key' and the item's
`label'. When this `.aux' file is read by the `\begin{document}'
command, the item's `label' is associated with `cite_key', causing the
reference to `cite_key' by a `\cite' command to produce the associated
`label'.