Page numbers do not appear on part titles. Most citation systems also call for the identification of the page number from which a quote or point is drawn. For example, such usage is specified in their citation formats of both the "Chicago Manual of Style", and "The Bluebook". E-books and other electronic documents published in a non-reflowable format such as PDF are normally paginated and numbered in the same way as their printed counterparts. While reading devices for reflowable documents such as EPUB e-books may display page numbers, these numbers change from device to device depending on factors such as the size of the display and the selected font size. This makes them unsuitable for citation purposes. To remedy this problem, Amazon Kindle e-books contain what are called "location numbers", that is, numbers in the margin of the electronic text that indicate where the corresponding page begins in the printed version of the book. In codicology, each physical sheet ("folium", abbreviated "fol." or "f.") of a manuscript is numbered, and the sides are referred to as "folium rectum" and "folium versum", abbreviated as "r" and "v" respectively. This results in designations like "5r" (the front side of the fifth sheet), or "8v" (the back side of the eighth sheet).