A class has many students. A student has many classes. In data modelling, this is a classic example of a many-to-many relationship. Type "Student = Class" into Dubbiya, my nascent browser-based, HTML5-dependent entity-relationship modelling tool, and until today, you got a Student entity, a Class entity, and a line between them.
The problem is, relational databases can't actually have real many-to-many relationships. You need to simulate them using an in-between table. Dubbiya now creates this for you:
This reflects my intention that Dubbiya be a tool for physical data modelling, not logical data modelling.