Back number (stylized as back number) is a Japanese trio rock band. The band’s record label is Universal Music. The group had its major debut in 2011. Its name is sometimes abbreviated to “Bakunan”, “bn”, or “bknb”. The official fan club name is “one room”. Their single “Christmas Song” (ja) reached number one on the Billboard Japan Hot 100 chart three times, twice in 2015 and once in 2016, and received a million certificate from the Recording Industry Association of Japan. The band’s first compilation album, Encore, reached number one on the Billboard Japan Hot Album for two consecutive weeks in 2016 and was the world best-selling album of the second week of 2017.
From wikipedia