def coordinate_frame_help(coord_map):
    """
    Return a help message explaining the contents of the coordinate
    frame measurement suffix mapping *coord_map*.
    """
    message = 'mappings between coordinate systems and measurement suffixes: '
    for frame, mapping in coord_map.items():
        message += ' [{}: north -> _{}, east -> {}]'.format(frame,
                                                            mapping['north'],
                                                            mapping['east'])
    return message