VMS Help EXCHANGE, INITIALIZE, Qualifiers, /DENSITY  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /DENSITY=density-value

    Specifies, for magnetic tape volumes, the density in bytes per
    inch (bpi) at which the tape is to be written.

    For magnetic tape volumes, the density value specified can be 800
    or 1600, as long as the density is supported by the magnetic tape
    drive. If you do not specify a density value for a blank tape,
    the system uses a default of the lowest density supported by the
    tape drive.

    For the RX02 dual-density diskette drive, use the DCL command
    INITIALIZE/DENSITY=SINGLE or INITIALIZE/DENSITY=DOUBLE to
    reformat the diskettes to a different density; then use the
    EXCHANGE command INITIALIZE to create the RT-11 directory
    structure.

                                   NOTE

       Diskettes formatted in double density cannot be read or
       written by the console block storage device (an RX01 drive)
       of a VAX-11/780 until they have been reformatted in single
       density.