def _normalize_units(units):
    """
    Convert units to normalized form.

    Converts to lower case and if necessary applies one of the following
    transforms:

        millim[.]* -> mm
        centim[.]* -> cm
        inch[.]*   -> inches

    """
    normed = units.lower()
    if normed.startswith('inch'):
        normed = 'inches'
    elif normed.startswith('millim'):
        normed = 'mm'
    elif normed.startswith('centim'):
        normed = 'cm'
    return normed
