Guided Tearing: What is the best way to cut a sheet of paper with a ruler?

We all know how to cut a piece of paper : hold it on a table firmly with a ruler, and

pull on one side along "an appropriate" direction.

Here we discuss the physics of fracture propagation in the challenging case of extremely bent thin sheets. Using a combined theoretical and experimental approach, we wish to determine the direction of fracture propagation, and to explain in which condition (pulling direction) the facture follows the ruler. Finally we answer the burning question of determining the best pulling direction (it slightly depends on the anisotropy of the material).