If the cost of a unit of each of ten articles be represented by the lines falling from the letters _A_, _B_, _C_, etc., to the base of the figure, a considerable fall in the cost of _A_ would put it below the cost of each of the other articles represented. If in the case of a large class of persons who did not formerly buy any of the _A_ it is as desirable as any of these goods, it will take its place as the most desirable subject of purchase instead of the least desirable. Those whose available means enabled them to acquire all the articles from _J_ to _B_ inclusive, but did not suffice for _A_, will now take the _A_ and omit the _B_. Those whose acquisitions stopped with _C_ will substitute _A_ for that article, and in general every buyer of any of these things who has not heretofore acquired _A_ will now put this in the place of the one which it was least worth while to acquire. [Illustration] _Substitutions caused by a Cheapening of one Utility in an Article which is a Composite of Several._--When different goods cost unlike amounts but are objects of equally strong desires, only one of them is a marginal purchase, and the others afford a personal gain to the consumer which is not offset by a cost. We have seen that this rule applies to the different utilities in a single good. In the case of every article several grades of which are sold, there is one component element or one utility which is worth to the buyer exactly what it costs, while the others afford a consumers' surplus. If the letters in the diagram represent, not whole articles, but utilities in articles, as discussed in Chapter VI, it will accurately express the essential facts. In such cases, which are very numerous, it is only necessary to reduce the price of the one utility which is now just worth its cost in order to induce more consumers to buy the grade containing this utility, instead of a lower grade of the same thing. In doing this, they forego the purchase of something else altogether, or content themselves with a lower grade of that other commodity. If jeweled watch cases should become cheaper, some persons would substitute them for plain cases and would forego buying, say, pictures which were just within their purchase limit, or would content themselves with cheaper pictures. This taking of one thing within the margin of consumption and discarding others is far less frequently done than is the taking of a lower grade of one kind of goods for the sake of securing a higher grade of another.