Abbott Handerson Thayer's correspondence consists primarily of letters to Thayer, but includes a few handwritten copies or drafts of his outgoing letters. Found here are numerous personal letters to and from family members, including his daughters Mary and Gladys (Galla), his son Gerald (Gra), and his wife Emma (also known as Addie), as well as nephews, nieces, sisters, cousins, and other extended family. There is correspondence with his patrons Charles L. Freer and John Gellatly; with many artists, several of whom were close friends, including Samuel Colman, Thomas Millie Dow, Daniel Chester French, Richard Meryman, Everton Sainsbury, Louis Agassiz Fuertes, and E. M. Taber; and former students, such as Ben Foster and Barry Faulkner; with other friends, many of them prominent members of society, such as Samuel Clemens, Royal Cortissoz, Edward Waldo Emerson, and Stanford White. A large amount of Thayer's correspondence concerns his research on protective coloration in nature and the publication of his book Concealing Coloration in the Animal Kingdom. To pursue and defend his theories on concealing coloration, especially as applied to birds, he corresponded with many notable naturalists, biologists, ornithologists, and collection curators. Most notable is his extensive correspondence with Sir Edwin Bagnall Poulton, a British zoologist, and draft letters to Theodore Roosevelt, in defense and promotion of his work. He corresponded with many, including Franklin D. Roosevelt, then the Assistant Secretary of the Navy, and the War Office in London, promoting his theory on the value of concealing coloration for warships and the military. Correspondence is arranged alphabetically by last name of correspondent and chronologically within each folder.

This subseries contains the correspondence of Emma Beach Thayer, also known as Addie by her family, and document her personal life, family relationships, and management of Abbott Thayer's legacy. Found here is a large amount of correspondence with family members, including her step-daughters Mary and Gladys, her step-son Gerald, as well as her parents, sisters, nephews, nieces, and other extended family. Also found is extensive correspondence with her friend, and Thayer's first wife, Kate Bloede Thayer. Much of her correspondence is with the same people who corresponded with Abbott Thayer, but also includes other friends, most notably, Henry Ward Beecher.

After 1916 Emma managed much of Abbott Thayer's correspondence due to his poor health, and included here are a few letters regarding the sale or exhibition of his artwork. Many of the letters found here are condolences to Emma after Abbott's death in 1921, including many letters from artists and friends expressing thoughts and memories of his life. There are also letters from Emma to many of Abbott's friends requesting his letters or copies of his for a planned book of Thayer's correspondence.

Researchers should note that correspondence between Emma and Abbott is arranged in subseries 2.1. Emma's correspondence is arranged alphabetically by last name and chronologically within each folder.

Kate Bloede Thayer's correspondence documents family activities and friendships during the 1880s. Included are letters in German from her grandfather, letters from her brother, Victor, and several letters from her daughter Mary sent during Kate's time at a sanatorium. Also found are letters from artist friends including Arthur Bell, Thomas Millie Dow, and Lewis T. Ives. Dow's letters discuss his travels in Europe, his current artwork, mutual friends such as Arthur Bell, and other general news. Ives's letters also discuss mutual friends and general news. Letters from Kate to her in-laws, William and Ellen Thayer, discuss family news and news of Abbot's work. Other correspondence, arranged alphabetically, contains letters from family and friends. Correspondence between Kate Bloede Thayer and Emma Beach Thayer is found with Emma Beach's correspondence.

The correspondence of Gladys Thayer Reasoner, a.k.a Galla, documents family activities and her friendships. Found here are letters to and from family members, including her sister Mary, her brother Gerald (known as Gra), nephews, nieces, and other extended family, such as her cousins in the Foster and Whiting families. Additional letters are from friends, including Randolph Bourne, Frances Burns, Evangeline Stillman, Blanche Walton, and others. Gladys also maintained correspondence with Thayer family friends, who were friends of her parents, or members of the Dublin, New Hampshire community in which they lived, such as Mary Amory Greene, John Gellatly, Richard Meryman, Raphael Pumpelly, and Jay and Philip Whelpley. Gladys' correspondence is also found in the subseries containing her father and stepmother Emma Beach Thayer correspondence.

David Reasoner's correspondence is primarily with family and friends, such as Emma Beach Thayer's sister, Violet, and from Mary Thayer Birch discussing Emma's estate, Gerald's ill health, and other family news. David Reasoner also corresponded with John Gellatly concerning the sale of Abbott Thayer's paintings, and with Gerald Thayer concerning financial matters. Other alphabetical correspondence is with Thayer and Reasoner family members, people working on books or requesting information on Abbott Thayer, museums exhibiting Thayer's work, and people interested in purchasing artwork by Thayer. Also found are letters concerning the deaths of Reasoner's mother, his wife Gladys, and his son Allen. The bulk of the David Reasoner correspondence concerning the estates of Abbott H. Thayer and Emma Beach Thayer is found in Series 4: Estate Papers.

This series contains the correspondence of other members of the Thayer and Reasoner families. Included is correspondence between Elsie Birch and her cousins, and the correspondence of her mother, Mary Thayer Birch. Also found are letters to Jean Reasoner Plunket, Abbott Thayer's granddaughter, including many from Nelson C. White, who collected research material for a biography of Abbott Thayer.

Gerald H. Thayer's correspondence discusses concealing coloration, the estate of Abbott and Emma Thayer, and letters of condolence from George Gray Barnard and Edwin H. Blashfield. Additional correspondence found here includes letters to Gerald's wife, Alma, from the family friend and Thayer patron, John Gellatly, letters to Abbott Thayer's parents, Dr. William and Ellen Thayer, and letters to Jay and Philip Whelpley, family friends of the Thayers.