Depending on the era in which a piano concerto was composed, the orchestra parts may provide a fairly subordinate accompaniment role, setting out the bassline and chord progression over which the piano plays solo parts (more typical during the Baroque music era, from 1600 to 1750 and the Classical period, from 1730 to 1800), or the orchestra may be given an almost equal role to the piano soloist, with orchestra instrumentalists and sections introducing key musical themes and playing virtuostic parts for their instruments and in which there is a "dialogue" or "conversation" between the piano soloist and the orchestra. When music students and music competition auditionees play piano concertos, the orchestra part may be performed in an orchestral reduction, a conversion of the orchestra parts into a part for an accompanist playing piano or pipe organ, as it is very expensive to hire a full orchestra. Keyboard concerti were common in the time of Johann Sebastian Bach in the Baroque music era, during the Classical period and during the Romantic music era (1800–1910). Keyboard concertos are also written by contemporary classical music composers. Twentieth- and 21st-century piano concertos may include experimental or unusual performance techniques. In the 20th and 21st century, J.S. Bach's harpsichord concertos are sometimes played on piano. There are variant types of piano concertos, including double piano concertos, for two solo pianists and orchestra, and double or triple (or larger solo groups) concertos in which the piano soloist is joined by a violinist, cellist, or another instrumentalist.