def get_format_vertical_parity_sign(format):
    """
    Get the vertical data layout associated with an image format, expressed as a
    coordinate system parity sign.

    Parameters
    ----------
    format : :class:`str`
        The format name; one of ``SUPPORTED_FORMATS``

    Returns
    -------
    Either +1 or -1, depending on the format's preferred parity. FITS has +1 and
    everything else has -1 (for now).

    Notes
    -----
    See :meth:`Image.get_parity_sign` for a discussion of general parity
    concepts. While parity should be associated with coordinate systems, rather
    than image file formats, there is a connection in the vertical data layout
    that different file formats are assumed to use. In particular, WWT assumes
    that most image file formats use a "top-down" data layout: if the data
    buffer is a 2D array expressed in standard C/Python conventions, the zeroth
    row (``data[0,:]``) is located at the top of the image. FITS formats, on the
    other hand, are assumed to be bottoms-up: the zeroth row is at the bottom.
    While proper WCS coordinates can convey *either* parity in a FITS file,
    WWT's renderer assumes bottoms-up.
    """
    if format == "fits":
        return +1
    return -1