The minus sign or hyphen (-) in place of a given name in any path
  name indicates a name one generation back.  This substitution is
  valid only for the first given names in the specification.  Once you
  have specified a given name, you may not use any hyphens further down
  in the chain:  the hyphens must be first in the string.

  For example, if your default directory is CDD$TOP.SALES.JONES, you
  can specify the CORPORATE directory by typing -.-.CORPORATE.  Note
  that -.SALES.-.PRODUCTION is not a legal usage because the sequence
  SALES.- is not allowed.