Johann Sebastian Bach's entire oeuvre is characterized by arrangements of his own works, the re-utilization of movements and a general policy of "material recycling", so that many works contain references to others. This is especially true of his concertos. Many of Bach's concertos have been lost or exist only in his own arrangements. Over and above his teaching duties, during his term as Thomaskantor and "Director musices" in Leipzig, Bach was primarily involved with delivering music for the Lutheran services at the city's central churches. But here too, he sought a sphere of activity that gave him the chance to perform instrumental music at secular venues. In 1729 he assumed direction of the students' Collegium Musicum, formed by his eminent predecessor Georg Philipp Telemann in 1704. The Leipzig collegium musicum was made up of professional musicians and students; it gave public performances once a week (more often during the Leipzig Fair) throughout the academic year; in the summer, they took place on Wednesdays from 4 to 6 p.m. at the coffee-house garden "before the Grimma Gate" and on winter Fridays from 8 to 10 p.m. at Zimmermann's Coffee House. Bach composed a great many important works for the collegium musicum; he very frequently adapted earlier compositions to suit the new conditions and available performing forces.