The upper part of a saxophone, located between mouthpiece and body, is called the neck (or bocal). On a tenor saxophone this is S-shaped while on the alto sax it is slightly angled. The neck is an essential component in terms of sound, response and intonation. That’s why some manufacturers offer necks separately to let you modify the instrument in specific ways.
These necks are made of different materials offering various sound characteristics. Before you start looking for a neck, you should know precisely which sound you are aiming for and that you have acquired a good embouchure – a term referring to position and use of your lips, tongue, and teeth when playing. Normally that takes about 2-3 years of playing. Just as we saw when choosing an instrument, the same rule applies here: "The proof of the pudding is in the eating!" Necks are not all suitable for every instrument. When fitting a neck becomes a problem, you should consult an instrument maker who has special tools for this purpose.