In general, photographers agree on the common idea that the rule of thirds is an essential part of photocomposition.
Well, it is, however, almost. In fact, this rule is wrong.
So let me limit the scope of this article to landscape orientation, rather than portrait orientation.
The idea is initially based on the “Golden Rule Of Proportions”, and the original formula is
x : y = y : ( x + y )
In numeric values, that translates like
5 : 8 = 8 : ( 5 + 8 )
5 : 8 = 8 : 13
Now, 5 is NOT one-third of