Why Is a Brass Saxophone Classed as a Woodwind?

A saxophone is usually made of brass and looks unmistakably metallic, yet it belongs to the woodwind family. The decisive feature is not the material but what first sets the air vibrating. When the player blows, a single reed fixed to the mouthpiece repeatedly opens and closes, regulating the airflow while the air column inside the tube forms the sound. On a trumpet or trombone, by contrast, the player’s vibrating lips initiate the sound. That is the more durable boundary between woodwind and brass.

The saxophone also joins its single reed to a tube that gradually widens into a cone. This differs from the clarinet, which also has a single reed but an approximately cylindrical bore. When a saxophone moves into its upper register it chiefly rises by an octave, and its colour remains comparatively connected across registers. Its brass body affects weight, resonance and details of timbre, but it does not change the underlying sound-producing mechanism.

When listening to an alto saxophone, it is worth moving past the question of whether it resembles brass or woodwind and attending to the start of each note. With gentle air, the reed can produce a grainy, subdued voice; with a more focused airstream and embouchure, the sound can quickly gain projection. This flexibility is not a vague mixture of two families. The single reed, conical air column and metal body each do a different job. Here, classification does not describe appearance: it identifies how sound begins.

https://www.britannica.com/art/saxophone
https://www.metmuseum.org/art/collection/search/504201
https://www.yamaha.com/en/musical_instrument_guide/saxophone/mechanism/mechanism003.html