X17 stands for an innovative, patented modular book system. And X17 stands for the 17-year warranty that the company gives on the book cover. Sounds incredible but it is true. When the company had produced the product, they tried to destroy it, but the leather remained intact. Of course everything can go wrong but then you have to go extreme. But with daily use, the material is indestructible. That's why it's called X17 (years).