Mouthpiece (brass)
The mouthpiece on brass instruments is the part of the instrument placed on the player's lips. The mouthpiece is a circular opening that is enclosed by a rim and that leads to the instrument via a semi-spherical or conical cavity called the cup. From the cup, a smaller opening (the throat) leads into a tapered cylindrical passage called the backbore. The backbore is housed in a tapered shank, which is inserted into an opening called the receiver on the main body of the instrument. On all brass instruments, sound is produced when the player's vibrating  lips (embouchure) cause the air column inside the instrument to vibrate. This is done by pressing the lips together and blowing air through them in order to produce a 'buzz.' The mouthpiece is where this lip vibration takes place. On most instruments, the mouthpiece can be detached from the main instrument in order to facilitate putting the instrument in its case, to use different mouthpieces with the same instrument, or to 'play' the mouthpiece by itself to exercise the player's embouchure. Different mouthpieces will produce different qualities of tone when used with the same instrument. Lower instruments also have  larger mouthpieces, to maximize resonance (see pitch of brass instruments). Also, mouthpieces are selected to suit the embouchure of the player, to produce a certain timbre, or to optimize the instrument for certain playing styles. For example, trumpet and trombone mouthpieces are usually semi-spherical whereas French horn mouthpieces are conical. Mouthpiece design. The mouthpiece has a large effect on instrument sound. Major effects are due to the shape of the cup, shape of the throat, and the inner rim diameter. In addition, players often choose a mouthpiece that complements their playing styles. In general, brass players who concentrate on  the upper range prefer a mouthpiece with a narrow bore, and players who emphasize the lower range prefer a wider bore. The cup depth also heavily influences the tone of the brass instrument. A shallow cup will brighten the sound, which is good for trumpeters doing a solo, while the deep cups darken the sound, which is better for blending in with an orchestra. Material. Makers commonly construct mouthpieces from one of two types of material, with different costs, properties, and features. Metal mouthpieces can be plated with some other metal. Some of the following assertions, especially those regarding the  effect of plating on tone color, are questioned by many players and specialists. Brass. Mouthpieces have traditionally been formed of solid brass. Due to brass often containing lead and being toxic upon contact, brass mouthpieces are usually plated with either gold or silver to protect the player from potential brass poisoning. Plastic. Plastic mouthpieces are usually made of Lexan plastic, and are often available in various colors. They are durable and don't dent as do metal mouthpieces. Less expensive than metal mouthpieces, players commonly use them when playing outdoors—particularly marching brass players—because their heat conductivity is far  less than in a metal mouthpiece. Players who prefer metal mouthpieces generally believe plastic mouthpieces have an inferior tone quality and feel compared to metal. Other. Recent additions to the mouthpiece world include stainless steel, titanium, and wood. They are relatively rare, produced by few manufacturers. Some players feel stainless steel and titanium mouthpieces provide advantages over the classic brass mouthpiece, including, anecdotally, a more centered feel and sound, as stainless steel and titanium do not absorb as many vibrations as brass, they require much less care, etc.—but they are much more expensive. (Titanium mouthpieces cost up to US$400  each.) Silver plating. Silver plating is common on all brass mouthpieces because it is cost-effective and good in terms of tone quality. It is also moderately germicidal. Silver plating is not as comfortable or as expensive as gold, but has properties and qualities that some feel facilitate certain styles of playing. Some believe that silver plate provides a clearer, darker sound than gold and is good for styles of playing that require clarity and projection. Silver-plate is less expensive than gold, but requires more maintenance because it tarnishes easily. Slightly tarnished silver-plate can be polished back to  its brightness with silver polish. Gold plating. Some players believe gold-plated mouthpieces on brass instruments create a fuller, richer tone that can also be somewhat darker timbre. For people allergic to the nickel found in most silver, this is the best (but not cheapest) way to play a brass instrument without discomfort. Gold does not tarnish, and subsequently requires little maintenance apart from regular washing with soap and water. The extreme price of gold, however, means that the plating is usually relatively thin and thus fragile, and can even be worn away with use. Sizes. Each mouthpiece company uses  a different labeling system. A larger number can mean a larger "or" smaller mouthpiece depending on the company. Likewise, the letters mean different things depending on the company. Even if companies appear to share marking systems it may be that same-marked mouthpieces from different manufacturers are different, although usually the differences are relatively small. There is no universally-recognized industry standard.