Liners are writing instruments similar to fountain pens, but with a much thinner and stiffer tip. They are used for precise and detailed lines in drawings, sketches, illustrations, and technical writing. Liners come in different tip thicknesses, usually expressed in millimeters, to meet the specific needs of the artist or writer. They may have permanent or washable ink and are appreciated for their ability to provide fine and clear lines without spreading or fading. Liners are commonly used in fields such as graphic design, architecture, illustration, and technical drawing.