For the past several posts I have been listing tetrads in juxtaposition to a set of graphics:
These four graphics are representative of the four fundamental relationships in the relational model using a notational style called IDEF1X. These four relationships are used to design relational databases. I am of the opinion that these relationships form what I call a “tetrad”, a term I borrow from Marshall McLuhan, and that it shares characteristics with other tetrads.
I invite you to look at the other posts under the tetrad category in my blog.