The concept of these German-made notebooks consists of a bonded leather cover, three elastic rings, and three Norwegian paper inserts that are endlessly changeable and refillable. The bonded leather covers that are so durable they come with a 17-year guarantee (hence the X17 name). The inserts are available in three different formats: lined, blank, and grid. Each notebook, available in two sizes, comes with three paper inserts—one of each format. Inserts contain 61 numbered pages.