Manava Dharmashastra 3.211

3.211

piturnāmasaṅkīrtanena tadīyāvāhanapiṇḍadānabrāhmaṇabhojanāni lakṣyante| kīrtayetprapitāmaham| jīvate pitāmahāya na dadyāt| kiṃ tarhi tataḥ pūrvābhyāṃ, ‘pituḥpitṛbhyo nivṛṇīyāditi' smaranti |

He whose father is dead, but grandfather is living, should mention the great-grandfather after having pronounced the name of the father.—(221)

Medhātithi’s commentary (manubhāṣya):
The pronouncing of the Father’s name stands here for his invitation, offering of the ball and the feeding of Brāhmaṇas.

‘Should mention the great-grandfather;’—i.e., he should not make any offering to the living grandfather; it should be made to his previous ancestors; as it has been declared that ‘one should make offerings to his father’s ancestors.’—(221)