"So Lonely" is a song written and produced by American singer Mariah Carey and record producer Rodney Jerkins. It is a duet between rapper Twista and Carey, featured on his album The Day After and Carey's reissue of The Emancipation of Mimi (2005), subtitled Ultra Platinum Edition. It was unofficially released in 2006 as the third single from The Day After. The version on The Emancipation of Mimi serves as a sequel track to "One and Only" (also featuring Twista), another track from The Emancipation of Mimi, and is subtitled "One and Only Part 2". The version on The Day After serves as a sequel track to that and is therefore subtitled "One and Only Part 3".