An original investment in bonds, compounds automatically at the purchase yield only until the funds are paid back in the form of coupons and finally principal. If future investment rates during the life of the bond are less than the purchase yield, then the realized compound yield for the whole life of the bond will be less than the purchase yield. 
15c from, what one has a legal concern in, compensation for loss, interest damage, it is of importance it makes a difference. Financial sense of "money paid for the use of money lent" (1520s) earlier was distinguished from usury by being in reference to "compensation due from a defaulting debtor. Interest rate by 1959.