A string trio is a group of three string instruments or a piece written for such a group. The term is generally used with reference to works of chamber music from the Classical period to the present. From at least the 19th century on, the term "string trio" with otherwise unspecified instrumentation normally refers to the combination violin, viola and cello (cf "string quartet", "piano trio" etc.). This is how the term is used by major catalogs, including IMSLP.