unitlength

Unitlength is a dimension used in TeX-based typesetting, most commonly in the LaTeX picture environment, to define the size of a single unit in the picture’s coordinate system. It establishes the scale for drawing elements by making coordinates and lengths in picture commands represent multiples of this base length. As a result, altering unitlength changes the overall size of a picture without changing the numeric coordinates used in the commands.

In practice, unitlength is set by the user to map picture units to a concrete physical dimension. In LaTeX, a typical statement is setlength{\unitlength}{1mm}, which means one unit in the picture equals 1 millimeter on the page. Thereafter, commands such as \put, \line, and \vector interpret coordinates and lengths in that unit system. For example, with unitlength = 1mm, a horizontal line drawn with \line(1,0){60} is 60 millimeters long.

The value of unitlength is specified in standard TeX dimension units, such as pt (points), mm, cm, in, as well as relative units like ex and em that reflect font metrics. It neither changes the typographic size of text nor the document’s general layout outside the picture environment; it only affects the scaling of graphics drawn inside picture.

Common considerations include choosing a unitlength that yields convenient drawing sizes for the intended document and ensuring consistency across multiple pictures. While the picture environment remains a lightweight option for simple diagrams, many users prefer modern packages such as TikZ or PSTricks for more complex illustrations.