The @STRING command is used to define abbreviations for use by BibTeX within the bibliography database file. The command
@string{jgg1 = "Journal of Gnats and Gnus, Series~1"}
defines 'jgg1' to be the abbreviation for the string "Journal of Gnats and Gnus, Series~1". Parentheses can be used in place of the outermost braces in the @string command, and braces can be used instead of the quotation marks. The text must have matching braces.
The case of letters is ignored in an abbreviation as well as in the command name @string, so the command above could have been written:
@STRING{JgG1 = "Journal of Gnats and Gnus, Series~1"}
A @string command can appear anywhere before or between entries in a bibliography database file. However, it must come before any use of the abbreviation, so a sensible place for @string commands is at the beginning of the file. A @string command in the bibliography database file takes precedence over a definition made by the bibliography style, so it can be used to change the definition of an abbreviation such as 'Feb'.