The best quality seems to have been sold in sheets about ten inches wide, the poorest that could be used to write upon, about six. The height in each case was perhaps one inch to two inches greater. It has been calculated that a single papyrus plant would make about twenty sheets of the size proportioned to its height, and this number seems to have been made the commercial unit of measure (_scāpus_), by which the paper was sold in the market, a unit corresponding roughly to our quire. [Illustration: FIGURE 189. INSTRUMENTS USED IN WRITING] §395. Pens and Ink.--Only the upper surface of the sheet was commonly written upon, the one formed by the horizontal layer of strips, and these showing even after the process of manufacture served to guide the pen of the writer. In the case of books where it was important to keep the number of lines constant to the page, they were ruled with a circular piece of lead. The pen (_calamus_) was made of a reed brought to a point and cleft much as our quill pens are. For the black ink (_ātrāmentum_, §391) was occasionally substituted the liquid of the cuttlefish. Red ink was much used for headings, ornaments, and the like, and in pictures the inkstand is generally represented with two compartments (Fig. 189). The ink was more like paint than modern ink, and could be wiped off when fresh with a damp sponge and washed off even when it had become dry and hard. To wash sheets in order to use them a second time was a mark of poverty or niggardliness, but the reverse side of _schēdae_ that had served their purpose was often used for scratch paper, especially in the schools (§110). §396. Making the Roll.--A single sheet might serve for a letter or other brief document, but for literary purposes many sheets would be required. These were not fastened side by side in a back, as are the separate sheets in our books, or numbered and laid loosely together, as we arrange them in our letters and manuscripts, but after the writing was done they were glued together at the sides (not at the tops) into a long unwieldy strip, with the lines on each sheet running parallel with the length of the strip, and with the writing on each sheet forming a column perpendicular to the length of the strip. On each side of the sheet, therefore, a margin was left as the writing was done, and these margins overlapping and glued together made a thick blank space, a double thickness of paper, between every two sheets in the strip. Very broad margins, too, were left at the top and bottom, where the paper would suffer from use a great deal more than in our books. When the sheets had been securely fastened together in the proper order a thin slip of wood was glued to the left (outer) margin of the first sheet, and a second slip (_umbilīcus_) to the right (also outer) margin of the last sheet, much as a wall map is mounted to-day. When not in use the volume was kept tightly rolled about the _umbilīcus_, and hence received its name (_volūmen_).