instrument. The slide section consists of a leadpipe, inner and outer slide tubes, and bracing, or "stays". The soldered stays on modern instruments replaced the loose stays found on sackbuts (medieval precursors to trombones). The most distinctive feature of the trombone is the slide that lengthens the tubing and lowers the pitch (cf. valve trombone). During the Renaissance, sleeves (called "stockings") were developed to decrease friction that would impede the slide's motion. These were soldered onto the ends of the inner slide tubes to slightly increase their diameter. The ends of inner slides on modern instruments are manufactured with a slightly larger diameter to achieve the same end. This part of the slide must be lubricated frequently. The slide section is connected to the bell section by the neckpipe and a U-bend called the bell or back bow. The joint connecting the slide and bell sections has a threaded collar to secure the connection. Prior to the early 20th century this connection was made with friction joints alone. Trombones have a short tuning slide in the U-shaped bend between the neckpipe and the bell, a feature designed by the French maker François Riedlocker in the early 19th century. It was incorporated into French and British designs, and later to German and American models, although German trombones were built without tuning slides well into the 20th century. Many types of trombone also include one or more rotary valves connected to additional tubing which lengthens the instrument. This extends the low range of the instrument and creates the option of using alternate slide positions for many notes. Like the trumpet, the trombone is considered a cylindrical bore instrument since it has extensive sections of tubing that are of unchanging diameter (the slide section must be cylindrical in order to function). Tenor trombones typically have a bore of (small bore) to (large or orchestral bore) after the leadpipe and through the slide. The bore expands through the bow to the bell, which is typically between . A number of common variations on trombone construction are noted below. Bells Trombone bells (and sometimes slides) may be constructed of different brass mixtures. The most common material is yellow brass (70% copper, 30% zinc), but other materials include rose brass (85% copper, 15% zinc) and red brass (90% copper, 10% zinc). Some manufacturers offer interchangeable bells. Tenor trombone bells are usually between in diameter, the most