In content for Windows and the Mac, use title capitalization for file names, folders, and drive names. Use all lowercase for extensions.

Microsoft style

.htm file

C:TaxesApril2010

My Tax File, 2010

It is all right to use internal capitalization for readability in concatenated file names if they cannot be confused with function names.

Microsoft style

MyTaxFile

Mytaxfile

Do not use the file name extension to identify the type of file under discussion unless you have no other choice. Use more descriptive language instead.

Microsoft style

A Microsoft Excel workbook contains one or more spreadsheets. Store the result as a text file.

Not Microsoft style

An .xlsx file contains one or more spreadsheets. Store the result as a .txt file.

If you must use the file name extension as a word, precede the extension with a period. If you must use an indefinite article (a or an), use the one that applies to the sound of the first letter of the extension, as though the period (or “dot”) is not pronounced, as in “a .com file” and “an .exe file.” In contexts that require title capitalization, capitalize as you would other common nouns.

Microsoft style

Initialization information is stored in an .ini file.

Save an .Rtf File as a .Doc File

Sometimes the file name extension corresponds to the abbreviation of the generic term for the file. For example, a .dll file contains a dynamic-link library (DLL). Do not confuse the abbreviation with the file name extension. For example, a .dll file (not a DLL file) contains a DLL.

When instructing users to type literal file names and paths, use lowercase for the file names and paths, and use bold formatting. Use italic formatting for placeholders.

Microsoft style

At the command prompt, type c:msmailmsmail.mmf password

For more information, see Capitalization (Chapter 7), Document conventions.