The Dynkin diagrams of type ADE are ubiquitous in mathematics as solutions of various classification problems. The diagram E<sub>6</sub> is usually drawn by five dots in a row with a sixth dot above the third, see [for example here][1]. There would be many other ways to draw   the diagram E<sub>6</sub>, for example the sixth dot below the five dots, or just a capital 

E.

Is there a reason for drawing that diagram in that particular shape beside or is that just a confirmed habit? 

  [1]: http://mathworld.wolfram.com/DynkinDiagram.html