A saxophone's serial number is typically located on the instrument's body tube, on the opposite side from the low D-key. Several models of saxophone also have serial numbers right below or right above the company logo on the bell.
Serial numbers can be used to determine when a saxophone was made or where and by whom it was manufactured. Model numbers, which are shorter and may have words in them, are not interchangeable with serial numbers. If a saxophone has a serial number on both its bell and its tube, the serial number on the bell generally takes precedence when attempting to date the saxophone.