def _backward_map(scale):
    """ Provide a mapping from plone scale names to collective.documentviewer
    scale names for convenience. This assures that code can use both naming
    conventions.

    :param scale: The name of a scale
    :type scale: A string
    :return: The corresponding name of the scale as understood by c.dv
    :rtype: string
    """
    if scale == 'thumb':
        return 'small'
    elif scale == 'preview':
        return 'normal'
    elif scale == 'large':
        return 'large'
    if scale not in ('large', 'normal', 'small'):
        scale = 'large'
    return scale
