The Custom of the Country has a few major geographical settings. Most of the story takes place in New York. Wharton carefully uses different areas of New York as settings to indicate differences in social class. The high society characters, the wealthy and fashionable, and all their social events are on Fifth Avenue. The Stentorian, where the Spraggs live, though, is a few blocks away, not quite up to the same level as the larger hotels and mansions. When Undine talks with Elmer in Central Park early in the novel, she intentionally meets him on the west side of the park, across the park from Fifth Avenue. Wharton carefully chooses settings within New York to clarify social relations in the novel.
There are also multiple European locales in the book, primarily Siena, where Undine and Ralph honeymoon, and Paris, where Undine and Ralph visit and Undine later lives with Raymond de Chelles. Most of the characters in the novel can move back and forth across the Atlantic fairly easily, using steam ships, then quite the engineering innovation. This ease of travel also shows the wealth of the novel's characters; by contrast, the reader is certain that Ralph is really running out of money when he tells Undine that they have to set sail for New York soon, because he can no longer afford rent in Paris. Most of the characters in the novel imagine Europe to be a more glamorous and admirable place than the United States. The exception to this is Undine, who becomes bored with both Italy (while on honeymoon with Ralph) and with France (while married to Raymond).
The last major setting is Apex. The midwestern city where Undine and Elmer grew up is only shown through memory and flashback, as well as through description by the characters themselves, but the reader never sees it firsthand. Wharton doesn't clarify what state Apex is in. But wherever it is, ironically, it seems far more difficult to reach from New York than Paris. Apex is presented as a geographical foil to New York, as the tiny town where the Spraggs, economically and socially, were well-known and prosperous. Apex was small enough that Elmer and Undine's elopement shocked the town. But when the Spraggs get to New York, Mr. Spragg's money stretches much less far, and Undine has to strive to be relevant on the social scene. Thus, Apex contrasts New York, despite never appearing directly in the story.