Vocal music can describe any music produced by the human voice, but is generally differentiated from choral music, which is sung by large groups. Because its instrument is the body itself, it is the most primal music produced by humans and is common to all cultures. It is most frequently expressed as song, words sung to a melody. At least since the Middle Ages, songs have commonly been accompanied by a string instrument, often played by the singer, a tradition that continues to the present. Art songs with piano accompaniment often gathered into thematically-related cycles, came to prominence in the 19th century in the works of Schubert, Schumann, Brahms, and Wolf. Nearly all European composers of the era wrote songs. Post-Romantic composers like Mahler and Richard Strauss popularized songs and song cycles with orchestral accompaniment, and songs have been significant in the output of modern composers such as Poulenc, Britten, and Barber.