noun
B-side; flip side of a record
Refers to the less-promoted side of a vinyl record or single; also used metaphorically for the lesser-known or secondary aspect of something.
このシングルのＢ面には隠れた名曲が収録されている。
The B-side of this single contains a hidden gem of a song.
彼の人生のＢ面はあまり知られていない。
The B-side of his life is not well known.