Design Principles - What is a line?

As I'm making a letter set for children, I have decided to research into interesting ways of folding letters.  Infographic versions of these could be printed on small card and placed inside the letter writing set.

 

http://www.origami-resource-center.com/letterfolds.html

Here is the kind of imagery I have been looking at:

http://imakecutethings.com/?p=98