Vaishnava Dharmashastra 75.4

75.4

yasya pitā pretaḥ syāt sa pitre piṇḍaṃ nidhāya pitāmahāt paraṃ dvābhyāṃ dadyāt ||

He whose father is dead (but whose grandfather is alive), must first of all offer a ball of rice to his father, after that, two balls to the two ancestors coming before his grandfather (or to his great-grandfather and to his fourth ascendant).