Improvements to instrument design and metal making in the late Middle Ages and Renaissance led to an increased usefulness of the trumpet as a musical instrument. The natural trumpets of this era consisted of a single coiled tube without valves and therefore could only produce the notes of a single overtone series. Changing keys required the player to change crooks of the instrument. The development of the upper, "clarino" register by specialist trumpeters—notably Cesare Bendinelli—would lend itself well to the Baroque era, also known as the "Golden Age of the natural trumpet." During this period, a vast body of music was written for virtuoso trumpeters. The art was revived in the mid-20th century and natural trumpet playing is again a thriving art around the world. Many modern players in Germany and the UK who perform Baroque music use a version of the natural trumpet fitted with three or four vent holes to aid in correcting out-of-tune notes in the harmonic series.