def thirds(a, b):
    """
    Return two points between a and b that split the line into thirds.
    Returns two values. The first is closer to a, and the second is closer to b.
    """
    aab = ( (2*a[0]+b[0])/3, (2*a[1]+b[1])/3, (2*a[2]+b[2])/3 )
    abb = ( (a[0]+b[0]*2)/3, (a[1]+b[1]*2)/3, (a[2]+b[2]*2)/3 )
    return aab, abb