Group portrait of Quatuor Pro Arte (Pro Arte String Quartet) in formal dress with their instruments. From left to right are Alphonse Onnou (first violin), Robert Maas (violoncello), Germain Prévost (viola) and Laurent Halleux (second violin). The quartet was founded in 1912 in Brussels by Alphonse Onnou. Germain Prévost and Laurent Halleux were original members of the quartet. Robert Maas joined the group in 1921. The photograph is inscribed to Lois Steers, and the date in the inscription is eight days after the quartet performed in concert at Portland's Public Auditorium.