Most trombone manufacturers engrave serial numbers onto the slide of the instrument, where it connects to the bell section. Occasionally, manufacturers engrave the serial number onto the slide lead pipe, where the mouthpiece fits into the instrument, or onto the bell.
Trombonists keep a record of the serial numbers to use in the case of theft. School-aged trombonists use serial numbers to avoid accidentally switching instruments. Serial numbers allow a trombonist to find the year the manufacturer made the instrument. Knowing the year of manufacture helps determine instrument value. On the Conn-Selmer website, the company lists serial numbers and manufacturing years for Conn, King and Vincent Bach trombones.