Their popularity in Spain was so great that the larger part of the nation's cork supplies went towards production of the shoes. Some argue that the style originated in Spain, as there are many extant examples and a great amount of pictorial and written reference going back to the 14th century.
The widespread popularity of this footwear style in Spain led to a significant portion of the country's cork supply being dedicated to its manufacture.  Some believe the style originated in Spain due to the abundance of existing examples and substantial historical evidence from the 14th century.