man tar

[Roderick]

We read there:

"""""
-f archive

Filename where the archive is stored. Defaults to /dev/rst0. If set to 
hyphen (‘-’) standard output is used. See also the TAPE environment 
variable.

""""""

Well, hyphen (‘-’) may also mean stdin as expected, but it seems not
to be mentioned/insinuated on the man page.

Rod.