Database normalisation is an approach to designing databases which was introduced by Edgar F. Codd in the 1970s. Certain databases, known as "relational databases", allow data to be stored in separate groups. Each group is commonly called a "table". To provide useful information, these groups are connected to each other. For example, students could be stored in one group, and classes in another group. To show that a student is enrolled in a class, a "relationship" is established from one group to the other. A student could have a relationship to many classes, each of which he or she would be enrolled in, while a class would have a relationship to many students.