3. _Degrees of power to affect price result from varying extent of control; monopoly is a relative term._ The term monopoly by its derivation has reference to a single seller; but there are other thoughts in the concept. Monopoly has reference also to the amount of the supply controlled. The frequent use of the adjectives partial, limited, and virtual are implied but usually superfluous recognitions of the relative character of monopoly. Ownership of a particular knife, pencil, book, makes one the unique seller of it, but confers no monopoly power, as the power of substitution is practically absolute; the welfare of no one depends in any appreciable measure on that particular pencil. Ownership of an important fraction of an entire species of goods gives more power to affect value. One owning a large part of the desirable building sites or houses in town may gain by occasionally letting one stand vacant in order to drive better bargains with tenants. A trade-union may control most of the labor-supply of one kind in a town. But the test of monopoly is that a gain results from a higher price and fewer sales. It begins at the point where there is a motive to limit the supply in accordance with the paradox of value. The control of an entire species of goods gives price-fixing power, limited only by substitution of goods. Even though one person controlled all the coal and wood in any market, their prices still would be limited. If there were but one possible source of meat-supply, most people could live without meat. The monopoly of great species of goods can thus be seen gradually to merge from one grade into another. It is a matter of quality as well as quantity. There is more or less of it in the different industries, and, as noted in the preceding paragraph, it varies over time and territory. § III. THE FIXING OF A MONOPOLY PRICE [Sidenote: Forces governing competitive prices]