def real_space_pixel_scales_tag_from_real_space_pixel_scales(real_space_pixel_scales):
    """Generate a sub-grid tag, to customize phase names based on the sub-grid size used.

    This changes the phase name 'phase_name' as follows:

    real_space_pixel_scales = None -> phase_name
    real_space_pixel_scales = 1 -> phase_name_real_space_pixel_scales_2
    real_space_pixel_scales = 4 -> phase_name_real_space_pixel_scales_4
    """
    if real_space_pixel_scales is None:
        return ""
    y = "{0:.2f}".format(real_space_pixel_scales[0])
    x = "{0:.2f}".format(real_space_pixel_scales[1])
    return "__rs_pix_" + y + "x" + x