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J.R.R. Tolkien and C.S. Lewis -

J.R.R. Tolkien, arguably the most iconic fantasy writer in history, popularized the genre during the 20th century with his massively successful book 'The Hobbit,' and the subsequent 'Lord of the Rings' trilogy. Naturally, Tolkien was close friends with perhaps the second most iconic fantasy writer in history: C.S. Lewis.

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J.R.R. Tolkien and C.S. Lewis -

C.S. Lewis cemented his name in the canon of fantasy literature with his seven-installment 'Chronicles of Narnia' series. The pair were best friends for more than 10 years and exchanged countless letters, until they began to grow apart around the 1950s.

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Agatha Christie and PG Wodehouse -

Agatha Christie remains to this day the most celebrated and prolific writer of mystery novels in history. Christie wrote 66 mystery books in her lifetime, as well as a play, 'The Mousetrap,' which remains the longest-running play in history. It seem only fitting that she would develop a correspondence with another master writer: PG Wodehouse.

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Agatha Christie and PG Wodehouse -

While Christie was the queen of mysteries, Wodehouse was the king of comedies. Famous for the now-archetypal characters Bertie Wooster and Jeeves, Wodehouse authored close to a hundred books in his life. Christie and Wodehouse didn't begin their correspondence until they were both of old age, and their warm letters seemed to keep them both company during their twilight years.

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Catherine the Great and Voltaire -

Catherine the Great, the long-reigning empress of the Russian Empire during the 18th century, is remembered for her active patronage of the arts. She also kept close a close correspondence with the French philosopher Voltaire.

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Catherine the Great and Voltaire -

Voltaire was a great admirer of the Russian empress, and although they never met, the pair exchanged letters for around 15 years. They shared a high level of mutual respect for each other. Voltaire praised Catherine as an "enlightened despot," and once deemed her "The Star of the North."

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Charles Darwin and Joseph Hooker -

Charles Darwin, the legendary biologist responsible for developing the revolutionary theory of evolution, often wrote to close friends and confidants to work out the very beginnings of his later earth-shattering ideas. One such friend was botanist Joseph Hooker.

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Charles Darwin and Joseph Hooker -

Joseph Hooker, famous in his own right for founding the field of geographical botany, is considered to be Darwin's best friend and encouraged his friend to develop his ideas. It was in a letter to Hooker that Darwin first let on about the early development of his theory of evolution.

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Jean-Paul Sartre and Simone de Beauvoir -

The relationship between philosophers Jean-Paul Sartre and Simone de Beauvoir, both as writing partners and as lovers, is just as famous as their respective contributions to literature and philosophy. They maintained a loving, open relationship for 51 years, without ever marrying, having children, or even sharing a house.

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Jean-Paul Sartre and Simone de Beauvoir -

While the pair had occasionally published a carefully selected and edited group of their many, many letters, it wasn't until Sartre's death in 1980 that Beauvoir published 'Letters to Sarte,' a huge collection of the couple's correspondence that shed light on the inner workings of their infamous and sometimes controversial relationship.

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Vincent van Gogh and Theo van Gogh -

While obviously best known for his work as a post-impressionist painter, Vincent van Gogh (pictured) was also a prolific writer of letters. He kept in very close contact with his younger brother, Theodorus van Gogh, who was more successful during their lifetimes and worked as an art dealer.

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Vincent van Gogh and Theo van Gogh -

Theo van Gogh supported his older brother financially during his life, and also took meticulous care to preserve all of the letters his painterly brother sent. Today, around 650 of the van Gogh letters are preserved, and regularly make rounds through the museum circuit.

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Vladimir Nabokov and Edmund Wilson -

 The prolific Russian author Vladimir Nabokov became close friends with critic Edmund Wilson shortly after arriving in the United States. The pair sent each other countless letters over the years of their tumultuous relationship.

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Vladimir Nabokov and Edmund Wilson -

More than 2,000 pages of correspondence between Nabokov and Wilson (pictured) exist, documenting the pair's discourses regarding various topics. The letters also document their eventual fallout, caused by growing ideological differences later on in life.

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Groucho Marx and T.S. Eliot -

One of the most surprising pairs of pen pal relationships from history is that between the iconic Jewish-American comedian Groucho Marx (pictured) and the infamously anti-Semitic poet T.S. Eliot. Despite their differences, the pair exchanged letters for around three years.

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Groucho Marx and T.S. Eliot -

Their correspondence started with Eliot writing a fan letter to Marx requesting a photo of the comedian. Marx obliged, and the two wrote tentatively affectionate, although sometimes backhandedly so, letters to each other until they finally met for dinner three years later. Although the specifics of the meeting aren't known, it must have been disastrous. The two never exchanged another word after that night.

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Henry James and Edith Wharton -

Two of the finest novelists of the 19th and 20th centuries, Henry James and Edith Wharton maintained a very close friendship over the years. James, author of 'Portrait of a Woman,' was a great admirer of Wharton's work, and frequently encouraged her writing.

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Henry James and Edith Wharton -

Edith Wharton, most famous for her novel 'The Age of Innocence,' was likewise a supporter of James' writing. Both writers suffered from a host of personal issues, Wharton with her troubled marriage and James with his debilitating depression. The two managed to support each other on a personal level through the warm and confessional letters.

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Elizabeth Bishop and Robert Lowell -

Elizabeth Bishop was one of the most influential poets of the 20th century, and was awarded the Nobel Prize for her work in 1956. Her writing was characterized by crystal clear and deliberate observations, and a complete rejection of the confessional styles that were popular at the time.

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Elizabeth Bishop and Robert Lowell -

Throughout her life, Bishop maintained an extremely close friendship with her contemporary, the Bostonian poet Robert Lowell. Despite their closeness, they rarely saw each other, but their friendship and mutual admiration is well-documented in their countless letters.

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Harriet Beecher Stowe and George Eliot -

Harriet Beecher Stowe (pictured) was one of the most important figures of the American abolitionist movement of the 19th century. In 1852, she published 'Uncle Tom's Cabin,' a seminal anti-slavery novel that showcased the horrors of slavery to a largely metropolitan audience that had little direct exposure to the evil, leading to an increase in abolitionist sentiment within white and affluent communities.

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Harriet Beecher Stowe and George Eliot -

In the middle of the 1800s, Stowe struck up a correspondence with the popular British author Mary Ann Evans (pictured), who went by the pen name George Eliot. The two, writing to each other with an ocean between them, were ardent supporters of each other's work and wrote many letters back and forth regarding literature, society, and religion.

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Anaïs Nin and Henry Miller -

Two of the most controversial and prolific writers of the 20th century, Anaïs Nin and Henry Miller naturally participated in a fiery affair in Paris during the 1930s. Whenever the two were apart, they would write to each other nonstop.

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Anaïs Nin and Henry Miller -

Both were married at the time, but neither seemed to mind very much. Miller was notoriously libertine regarding his five marriages, and Nin's husbands were never mentioned in her diaries with the same frequency that her lovers were. Their letters, some published, are some of the most intimate known love letters exchanged between two public figures.

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Johannes Brahms and Clara Schumann -

Two of classical music's most luminary figures, Johannes Brahms and Clara Schumann maintained a strained and intense friendship. Although there were passionate feelings of romance shared between them, a relationship was never realized, due in part to their shared loyalty to Schumann's husband, Robert, who was also Brahms' mentor.

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Johannes Brahms and Clara Schumann -

Eventually, the two moved to different cities, but they continued to write to each other on a regular basis. Schumann struggled with depression after the death of her husband in 1856, and Brahms would write her long and impassioned letters trying to help her see the beauty in life again.

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Nancy Mitford and Evelyn Waugh -

Nancy Mitford (pictured), a popular figure of London's high society in the 20th century, is famous for her witty works of post-war social criticism such as the novel 'Love in a Cold Climate,' published in 1949.

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Nancy Mitford and Evelyn Waugh -

Mitford was close friends and pen pals with another popular British writer at the time, Evelyn Waugh (pictured). Waugh was a troubled writer, deeply affected by his time spent serving in World War II, and their letters frequently regarded their displeasure with society.

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Albert Einstein and Sigmund Freud -

In 1931, between the recovery from World War I and the beginning of World War II, genius physicist Albert Einstein sent a letter to the psychoanalyst Sigmund Freud. He was searching for a greater understanding of humankind's tendency towards violence. 

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Albert Einstein and Sigmund Freud -

The pair began a correspondence on the subject, discussing human nature at length and pondering ways to avoid war and reduce the amount of violence in the world. 

Sources: (Mental Floss) (The Simple Things) (Literary Traveler) (Big Think) 

See also: Surprising celebrity friendships

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