extract a sub directory form a tar file

anyone know if it is possable to extract a subdirectory in a tar file.

IE

tarfile contains

parent dir
-sub dir A
-sub dir B

I want to extract sub dir B.

The following should work:

    tar xvf tarball subdirB

From a typical tar(1) man page:

x Extracts the named files from the tape. If a named file
matches a directory whose contents were written to the
tape, this directory is (recursively) extracted.
.....

thanks Murphy. i guess i should have read the man page a bit more carfully instead of scanning it.