def METHOD_NAME(stiffnesses):
    """
    Takes a stiffness tensor in standard (3x3x3x3) form
    and returns the Voigt form (6x6).
    """
    voigt_notation = np.zeros((6, 6))
    for p in range(6):
        i, j = voigt_index_to_ij(p)
        for q in range(6):
            m, n = voigt_index_to_ij(q)
            voigt_notation[p, q] = stiffnesses[i, j, m, n]
    return voigt_notation