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Nintendo has finally revealed the purpose of the Switch 2's C button, which will be used to activate the GameChat function

The C button, only briefly seen until today, has been the subject of much speculation among gamers as to its purpose and function. That said, Nintendo has taken advantage of its Switch 2 Direct to reveal what it is.
This button is used to launch the GameChat feature, which lets you broadcast your games while chatting with other players.
However, Gamechat will only be available to Nintendo Switch Online subscribers.