For example, 1999, the left is the beginning, which is applicable to instant annuities (see above), and the right is the end (applicable to ordinary annuity). It should be emphasized that the present value and final value of ordinary annuity are at the end of the period. Some people will wonder why the present value is the end of the period. Look at its concept. It is the present value of compound interest with equal amount of income and expenditure at the end of each period. There are two meanings here, one is the final payment, and the other is the present value of compound interest, that is, the discount of the final payment. Therefore, from the perspective of extension, the annual sinking fund and annual fund recovery amount, which are the inverse operation of the final value of ordinary annuity and the inverse operation of the present value of ordinary annuity respectively, should undoubtedly be at the end of the period. At the same time, there is no doubt about the present value of perpetual annuities at the end of the period.