Why are brass instruments classified by the player’s lips rather than by the material used to make them?

Brass instruments are classified by the vibration of the player’s lips against a mouthpiece rather than their construction material because material is an unreliable indicator of an instrument's function or acoustic properties.12

While the term "brass" suggests a metallic composition, many historical ancestors of modern brass instruments—such as the cornett and the serpent—were crafted from wood. Conversely, many modern woodwinds, such as flutes and saxophones, are constructed from metal. Because the material does not dictate how the sound is produced, classifying instruments by their physical makeup is considered unsuitable for systematic study.2

Instead, these instruments are more accurately defined as lip-vibrated aerophones. In this class, the player's lips act as the primary vibrating medium that sets the air column within the instrument into motion. This method of tone production provides a consistent, acoustically grounded way to categorize instruments regardless of their cultural origin or the materials used to build them.123