§4. _The Importance of being Unimportant_. The above contrast between cases in which variation is possible, and those in which it is not possible, is reproduced with a heightened significance when we turn back to joint demand. The cases are perhaps less common in which it is _impossible_ to alter the proportions in which different commodities are jointly demanded, but there are many cases in which it is not nearly worth while to do so (and this amounts to very much the same thing). Cases of this sort are especially likely to occur when we are dealing with a commodity which accounts for only a tiny fraction of the costs of the industry which is its chief consumer. Sewing cotton, for example, is jointly demanded, with many other things, by the tailoring and other clothing trades; but the money which these trades spend on sewing cotton is so small a part of their total expenditure, that no ordinary variation in its price is likely to make it worth while to study the ways and means of using it in smaller quantities. When sewing cotton is bought by the domestic consumer, considerations which are fundamentally the same, though somewhat different in form, point to a similar conclusion. It is thus very difficult to assign to sewing cotton a specific marginal utility. This difficulty is of great importance in connection with the possibilities of monopolistic exploitation. For it means that the demand blade of the scissors upon which we rely to cut off excrescences of price is blunted, and if accordingly the producers constitute a strong enough combination to control the supply blade, they will possess an unusual power of advancing their selling prices as they choose. I am far from suggesting that Messrs. J. & P. Coats are to be condemned as an extortionate monopoly. On the contrary, during 1919, when the profits in highly competitive industries like the main branches of the cotton and woollen trades, soared exuberantly, the record of this concern seems to me one of distinct moderation. But the present point is that they possess an exceptional _power_ to fix the price of sewing cotton as they choose, and that this is attributable in no small degree to the fact that sewing cotton constitutes an essential but relatively trifling item in the expenses of the processes in which it is employed.