A quartet is a group of four musicians who perform together, or a piece of music written for four performers. You might hear a string quartet (two violins, a viola, and a cello) playing classical music at a concert, or a jazz quartet with piano, bass, drums, and saxophone improvising together.

It captures something special about this size of group. Four performers can create rich, complex harmonies without anyone's part getting lost in the crowd. Each musician's contribution matters. In a quartet, you can't just blend into the background like you might in a full orchestra.

The word isn't limited to music. Four singers performing together form a vocal quartet, like a barbershop quartet singing close harmonies on a street corner. Writers sometimes use quartet for a series of four related books or movies, like a fantasy quartet telling an epic story across four volumes.

When you hear quartet, you're almost always talking about a collaboration where four distinct voices or instruments come together to create something none of them could make alone.