- You may have meant the Alt key on the keyboard.
For example, a relational database with a table "employee" could have attributes like "employee_id", "national_insurance_number", and so on. In this case, both "employee_id" and "national_insurance_number" serve as unique identifiers for a given employee, and could thus arguably be used for a primary key. Hence, both of them are called "candidate keys". If, for example, "national_insurance_number" was chosen as the primary key, "employee_id" would become the alternate key.