What are Grammar, Dialectic, and Rhetoric?

I recently found an excellent illustration of the Trivium developed by Mr. Sahs, of the Regent School in Austin, Texas.  The Trivium is made up of three essential elements: Grammar, Dialectic, and Rhetoric.  (To learn more about the Trivium, I invite you to  read Mr. Blakey’s “Why Only the Trivium” posts on this blog as well as the essay by Dorothy Sayers on the Classical Chrisitan topic page). 

This illustration looks at the life and history of Abraham Lincoln, drawing distinctions between each of the elements of the Trivium: