A pen is a common writing instrument that applies ink to a surface, usually paper, for writing or drawing. Early pens such as reed pens, quill pens, dip pens and ruling pens held a small amount of ink on a nib or in a small void or cavity which had to be periodically recharged by dipping the tip of the pen into an inkwell. Today, such pens find only a small number of specialized uses, such as in illustration and calligraphy. Reed pens, quill pens and dip pens, which were used for writing, have been replaced by ballpoint pens, rollerball pens, fountain pens and felt or ceramic tip pens. Ruling pens, which were used for technical drawing and cartography, have been replaced by technical pens such as the Rapidograph. All of these modern pens contain internal ink reservoirs, such that they do not need to be dipped in ink while writing. Types Modern Pens commonly used today can be categorized based on the mechanism of the writing tip and the type of ink: A ballpoint pen dispenses a viscous oil-based ink by means of a small hard sphere, or ball, which rolls over the surface being written on. The ball is held captive in a socket at the tip of the pen with one half exposed and the other half immersed in ink from the pen's reservoir. When the ball rotates, it transfers the ink - which wets the ball - from the reservoir to the external surface. The ball is typically under a millimeter in diameter and made of brass, steel, or tungsten carbide. The ink, due to its high viscosity, does not permeate through paper and does not leave the tip of the pen by capillary action. As such, a bare minimum amount of ink is dispensed, with the result that the writing dries almost instantly and ink lasts longer than it does in other types of pen. Ballpoint pens are reliable, versatile and robust, and are available for a very wide range of prices. They have replaced fountain pens as the most common tool for everyday writing. A gel pen works similarly to a ballpoint pen, in that it dispenses ink using a rolling ball held in the writing tip. However, unlike oil-based ballpoint pen ink, gel pen ink consists of a water-based gel that has a pigment suspended in it. Because the ink is thick and