Wharton's style in The Custom of the Country is intricate and detailed. Her sentences are typically long and ornate, and they are full of literary references and complex vocabulary. She uses elegant images with rich diction. Wharton, as a stylist, was part of the movement of Realism, which often used thorough, intricate sentences in an attempt to depict life truly. Wharton was also educated and spent her life in literary communities, which makes her writing more allusive and erudite. These long, subtle sentences also help enable the novel's sarcastic tone, hiding sarcastic jokes about Undine's personality or other aspects of high society.
Wharton also carefully adjusts for each character, creating individual dialects throughout the novel. Ms. Heeny, the Spraggs' masseuse, speaks with slang and brash certainty. Undine's energy and anxiety bring rapidity to her prose. Elmer is brash and confident, slipping in thoughtful jokes and speaking with conviction, which reflects in Wharton's prose as well. Wharton also depicts a particular high-society dialect, which is slightly different in each of the Fifth Avenue characters, including Poppingham, Fairford, and both van Degens. Each has a particular voice befitting their station and origin. Wharton varies her style carefully to help depict different characters.
An important feature of Wharton's style is the stream of consciousness. Wharton, as a transitional figure from Realism to Modernism, was one of the first writers to use this method. She uses it often with Ralph to depict the mental state of an especially emotional character. This use of stream of consciousness helps depict Ralph as a foil to Undine, showing how overly thoughtful he is, contrasting how she changes her mind very easily. This stylistic choice helps create this contrast, which elaborates the novel's characters and plot.