In The Custom of the Country, Edith Wharton presents the dynamics of the "who is who" in New York society at the beginning of the 20th century.
The title of the novel The Custom of the Country is allegorical to the saying "When in Rome, do as the Romans". In this case, "the custom" referred to the comings and goings of the ruling families of early New York City. The worth of each family was measured according to their connections to the founding families of what once was New Amsterdam. Therefore, the closest the family was to its dutch roots, the more "pure" its pedigree would be.
From the story, we learn that these families had a tendency to marry amongst themselves, and most of the marriages were to preserve their lineage as well as to continue the control that they exercised in parts of the city. In the story, we see the importance that these families exuded among the common folk, and how they would snub people outside of their immediate circle. In order to be able to enter the circle you had to either have money or be a part of the inner sanctum of the old country families. They also had a tendency to do everything in groups. When they were not all together in one group, they would divide into subgroups that will eventually meet up together again. They all kept tabs on each other, as well. It was like being part of a huge family whose links were heritage and wealth.
Undine Spragg, the main character, came from what could have been classified as "upper class" in the Midwest. However, nothing prepared her for the amount of courtesies and traditions that were so important to the society of New York. She basically had to change completely, including the way she dressed, the people with whom she mingled, and the manner in which she behaved among society in order to be at least considered "fit" to be among the Dagonets and every other ruling family.
In the end, Undine was never satisfied, which shows the shallowness of her life. She did not really belong to the group she so desperately tried to fit in. Nothing could ever please her. She was chasing waterfalls that reflected the fantasies she created about herself living the life of a New York socialite. In order to follow the custom of the country, the first thing you need is to be one of them. And she was not.