setlengthunitlength1mm

In LaTeX, the unit length used by the picture drawing environment is controlled by the length register \unitlength. The command \setlength{\unitlength}{1mm} sets one unit to be exactly 1 millimeter. This makes coordinates used inside the picture environment map directly to millimeters on the page, which can simplify creating diagrams with predictable physical sizes.

How it works: The begin{picture} command takes width and height in units of \unitlength. If you set \unitlength to 1mm, then a picture declared as \begin{picture}(40,20) will occupy 40 mm by 20 mm in the final document. Coordinates passed to \put or other drawing primitives are also in these units; for example, \put(10,5){...} places content at 10 mm from the left and 5 mm from the bottom. Line lengths and other dimensions inside the picture are interpreted in the same unit system.

Notes and caveats: The default value of \unitlength is 1pt in many LaTeX configurations, so changing it to 1mm changes the scale of the entire picture. TeX points differ from millimeters by about a factor of 2.845 (1 mm ≈ 2.845 pt). The unit length setting is global and should be set before the picture environment where it is used. While convenient for simple diagrams, the picture environment has limitations for complex graphics. For more features and flexible scaling, packages like TikZ are commonly recommended.

In summary, \setlength{\unitlength}{1mm} aligns the picture coordinate system with millimeter measurements, enabling more intuitive sizing of simple drawings.