By Dr. David R. Roisum. 
 
Curl is seen on a wide variety of chemistries including paper, film, foil, nonwovens, textiles and more.

What is common between ALL of the types of curl is that the top side of the web is different than the bottom side. What makes the different types of curl is the distinctly different ways the top of the web can be made different than the bottom.
 
Web201.68h: 
​https://www.youtube.com/watch?v=bxgwdVUWzH