A pica is a unit of measurement equalling 1/6 of an inch or 4.24 millimeters. A remnant of old-school typesetting, newspapers and magazine production rooms in the United States and United Kingdom still use picas as their unit of measurement. This can be seen in layout programs such as QuarkXPress and PageMaker, which allows designers to display rulers in inches, picas, ciceros and points. Many physical production rulers, called pica sticks in American newsrooms, also have these four units of measurements on them.
In layout design, a good rule of thumb is that every element should be set at least one pica away from every other element on the page. In other words, one pica should separate columns of text, one pica should set headlines away from body text, text should be one pica away from border lines, and so on.
Under the Anglo-Saxon system of typesetting, a point is defined as 1/72 of an inch, or .353 millimeters. A pica equals 12 points, which would also be the height of a twelve point font in typography using this system.