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The Big Bang Theory is an interesting exploration of friendship, both in how a larger friend group operates and how people within that group interact on an individual level. For years, The Big Bang Theory fans have largely argued that Leonard Hofstadter and Sheldon Cooper or Howard Wolowitz and Raj Koothrappali were the best friends on the series. We took a look at the interpersonal relationships more closely. We can argue that Sheldon Cooper and Penny were actually the best friends on The Big Bang Theory. 

Sheldon let Penny call him by a nickname 

Sheldon Cooper allowed very few people to call him by a nickname. His mother and sister, Missy Cooper, referred to him as Shelly. His grandmother, affectionately known as MeeMaw, called Sheldon Moon-pie. His friends and work colleagues, however, referred to him as Sheldon and only Sheldon. 

Penny helps Sheldon fall asleep at her apartmnt by singing 'Soft Kitty' to him in 'The Big Bang Theory'
Penny with Sheldon Cooper | Cliff Lipson/CBS via Getty Images

Penny, however, regularly referred to Sheldon as Sweetie, which he seemed to accept without question. The affectionate pet name showcased a closeness between the two that the series would highlight several times over the years. 

Sheldon told Penny things he didn’t tell anyone else 

Sheldon Cooper was never short on words. The eccentric physicist shared stories of his early life and was plenty boastful regarding his academic achievements. He, however, wasn’t too quick to share more sensitive information. He was open and honest with Penny about the things that hurt him and the things that scared him, though. 

Sheldon opened up about his father’s indiscretion. Penny was the only person Sheldon trusted with the story of his father cheating on his mother. He also explained to Penny why surprise parties were overwhelming and upsetting to him. While Sheldon was needy with his friends, he was only emotionally vulnerable around Penny and, eventually, Amy. 

Sheldon even referred to Penny as a ‘good friend’ early on in the show 

It took a lot for Sheldon to consider someone a “good friend.” While Howard Wolowitz spent every day with Sheldon for years, he was still slow to consider him a close friend. He considered Penny a close friend a lot quicker. 

Sheldon Coper, Howard Wolowitz, Raj Koothrappali and Leonard Hofstadter sit in Sheldon and Leonard's apartment in 'The Big Bang Theory'
Sheldon Cooper, Howard Wolowitz, Raj Koothrappali, and Leonard Hofstadter | Sonja Flemming/CBS via Getty Images
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3 Important ‘Young Sheldon’ Characters Who Sheldon Cooper Never Mentioned in ‘The Big Bang Theory’

In the season 3 episode, “The Bozeman Reaction,” Sheldon moves from Pasadena, California, to Bozeman, Montana, after a robbery shakes his belief system. When he leaves, he notes that he is leaving behind three good friends and one acquaintance. While it would have been easy to assume Penny, who had known Sheldon for the shortest period, was the “acquaintance,” it was later revealed that she was a close friend and Howard was the acquaintance in question.