def SG(rho, rho_ref=<NUM_LIT>):
return rho/rho_ref<EOL>
r'''Calculates the specific gravity of a substance with respect to another
    substance; by default, this is water at 15.6 °C (60 °F). For gases, 
    normally the reference density is 1.2 kg/m^3, that of dry air. However, in 
    general specific gravity should always be specified with respect to the
    temperature and pressure of its reference fluid. This can vary widely.

    .. math::
        SG = \frac{\rho}{\rho_{ref}}

    Parameters
    ----------
    rho : float
        Density of the substance, [kg/m^3]
    rho_ref : float, optional
        Density of the reference substance, [kg/m^3]

    Returns
    -------
    SG : float
        Specific gravity of the substance with respect to the reference 
        density, [-]

    Notes
    -----
    Another common reference point is water at 4°C (rho_ref=999.9748691393087).
    Specific gravity is often used by consumers instead of density.
    The reference for solids is normally the same as for liquids - water.

    Examples
    --------
    >>> SG(860)
    0.8608461408159591