[X X] is a repackage of LOONA's debut mini album + +. It was released on February 19, 2019 with "Butterfly" serving as the album's title track.
The physical album comes in four versions: Normal versions (A & B) and Limited Editions (A & B).
On international digital platforms, the album was released as an EP featuring only the six new songs.
LOONA began teasing their comeback with the release of a teaser video titled "X1X" on January 1, 2019. This was followed by "XIIX", on the 11th, "XIIIX" on the 21st, and "XIVX" on the 31st.