common being sizes from . The smallest sizes are found in small jazz trombones and older narrow-bore instruments, while the larger sizes are common in orchestral models. Bass trombone bells can be as large as or more, though usually either in diameter. The bell may be constructed out of two separate brass sheets or out of one single piece of metal and hammered on a mandrel until the part is shaped correctly. The edge of the bell may be finished with or without a piece of bell wire to secure it, which also affects the tone quality; most bells are built with bell wire. Occasionally, trombone bells are made from solid sterling silver. Valve attachments Modern trombones often have a valve attachment, an extra loop of tubing attached to the bell section and engaged by a valve operated by the left hand by means of a lever or trigger. The valve attachment aids in increasing the lower range of the instrument, while also allowing alternate slide positions for difficult music passages. A valve can also make trills easier. The valve attachment was originally developed by German instrument maker Christian Friedrich Sattler in the late 1830s for the (), a B♭ tenor trombone built with the wider bore and larger bell of a bass trombone that Sattler had earlier invented in 1821. Sattler's valve attachment added about of tubing to lower the fundamental pitch from B♭ to F, controlled by a rotary valve, and is essentially unchanged in modern instruments. Valve attachments are most commonly found on tenor and bass trombones, but they can appear on sizes from soprano to contrabass. Soprano In the early 2010s Torbjörn Hultmark of the Royal College of Music commissioned the first soprano trombone in B♭ with an F valve, built by Thein Brass. Alto Although rare on the E alto trombone, a valve attachment usually lowers the instrument a perfect fourth into B, providing the first five or six positions from the tenor trombone slide. Some alto models have what is called a trill valve, providing a small loop of tubing that lowers the instrument by only a minor or major second, into D or D♭ respectively. Tenor Tenor trombones, especially the larger bore symphonic models, commonly have a valve attachment which lowers the instrument from B to F. It provides access to the otherwise missing notes between the pedal B♭ in first position, and