To correspond with the model of RFCs as books, RFCs are mostly paginated documents. For historical reasons, they are usually in ASCII and appear in a fixed-width font. Pages have a line of header and a line of footer. A form feed (x0C) character separates pages, and figures are drawn by "ASCII art." RFCs can also be in Postscript (.ps) or PDF (.pdf); for standards-track RFCs, an authoritative ASCII (.txt) version must exist.