I know we ALL know it, but straights against curves is always thrown into the ears of my students constantly. I can't emphasize it enough! I believe it's one of the cornerstones of art and design. In my small example here I took one of my sketches from my class and drew over it with various lil' examples of where this applies.
To say straights against curves may only conjure up one design to the theory. But if we say straight "AND" curves it may open up our mind to more variation.
Let it be a psychological "click" in your mind! Straights "AND" Curves!