Canon.

 What is a canon?

A canon is a collection of books or volumes of work. Law, rules by which something is judged. A collection of works which are considered to be accepted by all.

in photography, the specific canons are

1 1826 to 1902

2 Post secession 1902-1950

3 Contemporary 1950-now

4 Now. 

The key concepts of photography are.

1 History

2 Landscape

3 Portrait

4 Still life.