What is the equi-marginal principle?

What is the equi-marginal principle?
We will use the utility theory to explain consumer demand and to understand the nature of demand curves. For this purpose, we need to know the condition under which I, as a consumer, am most satisfied with my market basket of consumption goods. We say that a consumer attempts to maximize his or her utility, which means that the consumer chooses the most preferred of goods from what is available. Can we see what a rule for such an optimal decision would be? Certainly I would not expect that the last egg I am buying bring exactly the same marginal utility as the last pair of shoes I am buying, for shoes cost much more per unit than eggs. A more sensible rule would be: If good A costs twice as much as good B, then buy good A only when its marginal utility is at least twice as great as good B's marginal utility. This leads to the equimarginal principle that I should arrange my consumption so that every single good is bringing me the same marginal utility per dollar of expenditure. In such a situation, I am attaining maximum satisfaction or utility from my purchases. This is clear concept of equimarginal principle.