Much of the family correspondence is between Daniel Putnam Brinley and Kathrine Sanger Brinley during the periods that they were apart, especially the period before their marriage in 1904 and during the time that Daniel Putnam Brinley was in the American Expeditionary Forces in France from 1917 to 1919. These intimate, detailed letters discuss their travels and events in their everyday lives. Researchers should note that the Brinleys often addressed each other with nicknames such as "Jock" or "Put" for Daniel Putnam Briley, and "Kat" or "Dee Dee" for Kathrine Sanger Brinley. Also found are transcripts and partial transcripts of some of these letters. Additional family correspondence is between Kathrine Sanger Brinley and her sister Emma Sanger, often called "Gippie", "Gyppie", or "Gip", correspondence with her brother William Sanger, her parents William and Kate, and correspondence between Daniel Putnam Brinley and his mother. Within these letters the family discusses their travels, including the Brinley's extensive travels throughout Europe, and news of acquaintances and events. Also found here are a few of Elizabeth Loder's notes on the family correspondence.

General correspondence includes Daniel Putnam Brinley and Kathrine Sanger Brinley's personal and professional correspondence, mostly incoming letters, for a period of over 75 years. The bulk of the letters are from friends and extended family and discuss travel, mutual acquaintances, social events, and other general news. Much of the personal correspondence does not include the sender's last name. Also found are many letters from Katherine's former priest, E. A. Bazett-Jones, who often addressed her as "daughter" in his letters, and other members the religious community in which the Brinley's were active members.

Professional correspondence discusses Daniel Putnam Brinley's art exhibitions, mural and painting commissions, the sale of his artwork, and his involvement in art organizations and events. Also included is Kathrine Sanger Brinley's correspondence with magazine and book publishers such as magazine and Dodd, Mead & Company, regarding the publishing of her articles and travel books. The Brinley's maintained very little correspondence with other artists, but notable correspondents include the artists William Chadwick, Anthony Euwer, Charles H. Davis, Edward Bruce, Esperanza Gabay, Robert Henri (one letter, dated 1914), Hildreth Meiere, Edwin Blashfield, William A. Coffin, Ernest Peixotto, and Hugh Troy, as well as the writer John Erskine. This subseries also includes a small amount of Elizabeth Loder's correspondence with art organizations, galleries, and others regarding the Brinleys.

Researchers should note that additional correspondence can be found in Series 4: Commission Files and Series 5: Organization Files.