Illustrating many-to-many Relationships in a Database
Many to many relationships can only exist while you conceptually design the database and this is when you need to eliminate them. You physically cannot have it in a working database otherwise your schema would need to change continually.
A many-to-many (M:N) relationship, sometimes called non-specific, is when for one instance of entity A, there are zero, one, or many instances of entity B and for one instance of entity B there are zero, one, or many instances of entity A.
An example is:
"employees can be assigned to no more than two projects at the same time; projects must have assigned at least three employees"