2. _The sale of rent-charges grew out of an industrial need of the exchange of safe permanent incomes for larger sums of wealth._ The custom of the purchase of rent-charges grew up in the cities. The increasing wealth of cities, the growth of commerce and enterprise, caused rent-charges to be sold by the owners of houses and real estate in the cities, and the custom spread to the country. It is an instance of the way income became more fluid in the cities during the Middle Ages. This kind of loan contrasted strikingly in the Middle Ages with those loans made commonly by reckless kings, prodigal nobles, and distressed peasants to secure consumption goods. Merchants needed large amounts of wealth for their growing enterprises, and they felt that if they could get a capital sum down they could make it earn more than the rent-charge. A perpetual income of one hundred units was therefore exchanged for a sum at the moment of twenty or twenty-five times that amount. As the wealth of the cities increased, there were some men who wished to retire from active business, and there were widows and children with property which they could not manage directly. Such persons either could not afford to take the risks of active business, or could not judge of them, and they formed a class of lenders or investors seeking some safe income. Between the two classes of active merchants and capitalist lenders, each of whom saw his own advantage and followed it, the practice of buying and selling rent-charges thus grew up. [Sidenote: Rent-charges were not forbidden by the church] The practice was allowed by the church, though interest and the lending of money were forbidden. The loan was substantially a loan of capital and the rent-charge was substantially interest, but in the eyes of the church moralists there was a marked difference, in that the obligation to the purchaser of the rent-charge was secured by a permanent and substantial form of wealth, and the contract usually was favorable to the borrowers. In its origin the practice was not merely an evasion of the law against usury, but a convenient form of contract. It doubtless came, however, to be used as a means of evading the law of the church against usury, and thus became an entering wedge for the general use of money loans. [Sidenote: The market value of rent-charges reflects the exchange ratio between present and future money incomes]