One of the most fun things about K-pop and J-pop is the sub-unit. For the unaware, a sub-unit is when select members of a group form their own sub-group, usually in order to show off a different sound or just to highlight certain members. Earlier this year, Girls' Generation had enormous success with their own sub-unit, Girls' Generation-TTS, which focused primarily on powerful vocals instead of the slick choreography that the Divine Nine are known for. Sub-units aren't usually a regular thing, though, but come January, we're going to see unit projects from three of K-pop's biggest groups.

First is SISTAR, who will be reviving their SISTAR19 sub-unit with another Brave Brothers produced single. Comprised of power vocalist Hyorin and sexy rapper Bora, SISTAR19 first launched in 2011 with their smash single, "Ma Boy." The bubblegum banger was one of the hugest hits of the year, with the music video racking in over 22 million views, while the dance practice video has over 40. In the two years since SISTAR19 debuted, SISTAR has grown into one of the most successful girl groups in K-pop, so their unit comeback already looks like it's going to be massive.

Competing against SISTAR is rival group, 4minute, who are preparing to launch their first sub-unit, Double Yoon, next month. Not a whole lot is known about Double Yoon yet, but since the duo is made up of 4minute's two best vocalists, Jiyoon and Gayoon, we're guessing that there will probably be a lot of big vocals involved in their music. Chart wise, expectations aren't super high for Double Yoon -- HyunA is really the only popular member of 4minute, and the group's other members have been pushed further into the background since her solo career took off. But with the talent of Jiyoon and Gayoon, and the amazing music that Cube Entertainment churned out this year for B2St, HyunA, 4minute, and Yoseob, Double Yoon could turn out to be the dark horse of K-pop sub-units.

Outside of the ladies, awesome boy band, INFINITE, will be launching a hip-hop sub-unit called INFINITE H. Just like SISTAR, INFINITE have grown to become one of the biggest acts in K-pop over the past year or so, and they're also recognized as one of the most talented, so INFINITE H could really dominate the charts when they debut. Especially if the success of BIGBANG's GD&TOP is anything to go by.

Which sub-unit are you looking forward to the most this January?