Transference of merit
Transference of one’s merit as a way of making merit should be understood in the case of one who made a gift or offering and transfers his accumulated merit thus: ‘May this merit be for so and so!’ or ‘May it be for all beings!’
Now, will there be a loss of merit to him who thus transfers it? No. Just as when one lights an oil lamp and with it lights a thousand oil lamps, it cannot be said that his original lamp is exhausted. On the contrary, the light of his original lamp being one with the light of the thousand oil lamps increases immensely. Likewise, there is no decrease of merit to him who transfers it thus; on the contrary, there is an increase only.
Under this category would fall the transference of merit to the dead. Thus, The Buddha declares that one of the duties of children towards their dead parents is to transfer merit to them.