The class of Complementary goods, to use Menger’s term, is much wider than we are apt to suppose. In consumption goods, it tends to increase with the variety of modern wealth and the development of new tastes. Many of our enjoyments depend on the co-operation of a great many factors, of which usually one is prominent, and the others only assert themselves on rare occasions. Thus the part played by that insignificant commodity, salt, in most of the pleasures of the table, is never appreciated till the want of it—say, at a pic-nic—suggests how indispensable a complement it is. Among productive goods, again, where the division of labour is constantly adding to the number of factors which work together in the making of any good, the complementary character becomes even more apparent. The first thing to be noticed here is that the value of a group, _as a group_, is determined by the marginal utility of the group, not of the separate members. But, as each group may on occasion be broken up, the interesting question is as to the distribution of value among the members, the difference in value between goods as complements and goods as isolated articles. The simplest case is where the single members of a group are all useless in any other form but that of the group, and are at the same time economically irreplaceable. In valuing boots, for instance, the “good” is the pair; if I lose one, I lose the entire utility. In such cases—which are, of course, comparatively rare—if I have had the pair and lose one, I lose the entire value of the pair: if I have one and obtain another, I gain the entire value of the pair. Here, then, the value of one single member of the group is the same as the value of the whole group. This case, however, is really of importance only as introducing the others which follow; under the assumed conditions we are dealing with a good similar, say, to a pair of compasses or a pair of spectacles, which we can divide into two only at the cost of the whole; that is to say, it is only externally a group.