An identifying character string used to refer to a file. The name can be generated by software or created by the user. It usually gives some hint as to the contents of the file. Different operating systems have different rules and conventions for file-name construction. Often the rules allow (or require) the final section of the name (the file extension) to be separated by a period (full stop) and used to indicate the type of file. Sometimes upper and lower case letters are not distinguished. The maximum length of the name is often strictly enforced. Usually all letters and digits are allowable together with some special characters.