When a song is performed a cappella, it is sung unaccompanied by instrumental music.

// A hush fell over the audience as a voice from offstage began singing a cappella.

// Several a cappella groups are slated to perform during the celebration.

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"In a video posted to Twitter ... H.E.R. delivers a hauntingly beautiful cover of Coldplay’s classic 'Fix You,' which peaked at No. 59 on the Billboard Hot 100 in 2005. Backed by four of her background singers, H.E.R. belts out the 2005 hit completely a cappella." — Kyle Denis, Billboard, 14 July 2022

A cappella arrived in English in the 18th century via the Italian phrase a cappella, meaning "in chapel or choir style." (Medieval Latin capella, meaning "chapel," is the source of English chapel.) The a cappella style reached preeminence in the late 16th century in the music that composer Giovanni Pierluigi da Palestrina wrote for the Sistine Chapel of the Vatican. Because no independent instrumental parts were written down, scholars once thought that the choir sang unaccompanied, but current evidence makes clear that an organ or other instruments doubled some or several of the vocal parts. Regardless, today a cappella describes a purely vocal performance.