Classical music has come to refer to a canon, or a body of works accepted over time as worthy of study and performance. Originally, music created for the European aristocracy at court and in church, as opposed to ballads and comic songs performed by minstrels for common folk. By the 18th Century, the style was associated with more or less standardized instrumentation and accepted principles of counterpoint as exemplified by the composers Mozart and Haydn. Classical style includes this so-called Classical Era but also the preceding Baroque period of Bach and Handel and the subsequent Romanticism of Beethoven and Schubert. Other subgenres include 20th century post-Romanticism and 12-tone music, and some writers would also include early music from the Renaissance and the Middle Ages. The style includes vocal music such as opera, lieder and oratorio as well as instrumental music. Although classical music is usually intended to be performed as written, it also includes semi-improvisational experimental, electronic and aleatoric music often associated with universities.