The accumulation of a nation's capital and its investment require the cooperation of numerous agencies of which banks are the chief. They collect the savings of the people, combine them into amounts of sufficient size for investment purposes, and invest them temporarily and sometimes permanently. Cooperating agencies in this work are insurance companies, societies of various kinds for the promotion of saving, stock exchanges, promoters, etc. Some of these take the place of banks in the performance of these services, while others supplement and aid them. _3. Classification of Banking Institutions_ Banks differ from one another chiefly in the nature and degree of their specialization, in legal status, and in the place they occupy in the system to which they belong. Some banks devote the major portion of their effort to the conduct of exchanges and are called _commercial_ banks, others to investment banking and are called _investment_ banks. The most common subclasses under the latter head are savings banks, land or mortgage banks, and bond houses. Savings banks specialize in the collection and investment of small savings; land banks are primarily intermediaries between capitalists and people who wish to invest capital in land, building operations, and agriculture; and bond houses are intermediaries between capitalists and those who wish to invest capital in industrial, commercial, and transportation enterprises, or loan it to states, cities, or other public corporations. Commercial banks rarely confine themselves exclusively to the conduct of exchanges. Most of them also conduct savings departments and invest the funds intrusted to them through such departments in agricultural, industrial, or commercial enterprises or loan them to public corporations. Commercial banking, however, is their main concern, their other departments being side issues of greater or less importance according to circumstances. Investment banks also frequently carry on commercial banking as a side issue. These two lines of business are sometimes mixed in such proportions as to render classification difficult. From a legal point of view the banks of nearly all countries may be classified as _private_ or unincorporated, and _incorporated_, sometimes also called joint-stock banks. Private banks are started by individuals or firms, like any other private enterprise, without the formality of application for permission to some public officer, and without compliance with a set of legally prescribed regulations. They are subject to the laws of the country governing all kinds of private business enterprises and sometimes to special laws applying specifically to them. In some of the states of the United States such banks are prohibited by law.