After the assessment has been completed, it is comparatively easy to make the collection. All that is needed is a collecting agent of the treasury conveniently located, to whom the tax-payers may go, or a collector who goes to the tax-payers. The burden of taxation may be seriously increased if the convenience of the tax-payers is not consulted in this matter. The size of the district over which a collector has supervision will depend upon the density of the population. If the collector is to be sought out by the contributors, it is best that his office should be located in some business centre frequently visited by the contributors. According to the principle of " certainty and convenience," the taxes assessed upon the same person should all be entered in a single bill and all be payable to the same collector. The tax-payer should be informed as early as possible of the total amount of taxes that he has to pay, of the number of instalments in which they are payable, and of the conditions of delinquency and its penalty. Some of the American commonwealths disregard this rule entirely. They add grievously to the burden of taxation, especially in the country districts, where they are already entirely out of proportion to the ability of the people, and increase the irritation felt by the contributors, by inconvenient location of the collectors' office, and by requiring the payment of State and county taxes to one set of collectors, while the town and other municipal taxes are paid to a different set and upon separate bills. The most economical and least irritating process is to have all the taxes collected by the same person.1