Most brass instruments use a set of valves. When these valves are depressed, they produce different pitches. Most modern brass instruments excluding the trombone use valves, including the sousaphone, flugelhorn, trumpet, horn, euphonium, baritone horn, tuba, and the cornet.
Slide brass instruments use a slide that is moved in and out to change the pitch. The primary instrument in this category is the trombone.
With natural brass instruments, it's only possible to play notes in the instrument's harmonic series. The bugle is a prime example of this. Typically, natural brass instruments are usually only played for ceremonial functions and some period performances.
The vast majority of brass instruments have a removable mouthpiece. Different sizes, shapes, and styles of mouthpiece can be used to suit different embouchures (the tension of the lips on the mouthpiece), or to more easily produce certain tonal characteristics. Generally, children shouldn't start to learn a brass instrument until age 11, by which time their permanent teeth should be in. Before age 11, most children’s teeth are still changing which means that the embouchure will need to be adjusted regularly.
In the case of brass instruments, the sound is produced by the player buzzing his or her lips into a mouthpiece. One can change the pitch by altering muscular contraction in lip formation. The way in which you tighten your cheek and jaw muscles, as well as manipulation of your tongue, can affect the embouchure. When you first try it, the result could be anything from a low to high-pitched musical note, depending on the tension in your lips. When learning a brass instrument, you must learn to control these muscles.