It often happens that a monopolist can, by changing slightly the character of his product, supply different demands at different prices. Recurring to our soap manufacturer, let us suppose that instead of making only one kind, as we have assumed, he makes five kinds differing only in shape, color, or scent. It is entirely probable that he could sell his output at five different prices; those most highly scented, let us say, at the highest price, and so on down until the lowest price is reached with the unscented cakes. More familiar illustrations of this principle are found in the publication of certain kinds of books. Ordinarily, the earlier editions of books of fiction are sold at high prices. Later so-called popular editions are issued at half price or even less. The reading material is exactly the same, and occasionally no great change is made in the quality of paper or binding. Publishers know that there is a demand for the book at the standard price. They know also that many readers do not have demands at that price. Hence they supply each demand successively, beginning at the top, unless, as is sometimes the case, they make a show of supplying several demands at once by issuing different editions that appear to be very much unlike in printing and binding. Any peddler knows that in many cases the only way to sell an article at a discount to one housewife without losing the custom of her neighbors is to mar it in some manner so as to decrease its apparent utility. To use a common business expression, "one of the tricks of the trade" which the monopolist must learn is to be able to classify rather accurately the different demands for his product and to meet these demands.