@staticmethod
def _join_bc(sub1, sub2, axis=None):
    """ Determinae bc for the joined Subdomain. """
    if axis == 0:
        if sub1.axis == 0 and sub1.ix[0] < sub2.ix[0]:
            return f'{sub1.bc[0]}.{sub2.bc[2]}.'
        if sub1.axis == 0 and sub1.ix[0] > sub2.ix[0]:
            return f'{sub2.bc[0]}.{sub1.bc[2]}.'
    if axis == 1:
        if sub1.axis == 1 and sub1.iz[0] < sub2.iz[0]:
            return f'.{sub1.bc[1]}.{sub2.bc[3]}'
        if sub1.axis == 1 and sub1.iz[0] > sub2.iz[0]:
            return f'.{sub2.bc[1]}.{sub1.bc[3]}'
    return sub1.bc
