Which of the following is NOT a rule for dimensioning?

The answer highlights that each dimension should be shown only once to avoid confusion and ensure clarity in technical drawings. Dimensioning is an essential aspect of graphic communication, and clear, concise representation is vital for conveying precise measurements. Showing a dimension multiple times could lead to misinterpretation or misunderstanding, as it might suggest multiple measurements rather than reinforcing a single value.

In technical drawings, it's best practice to ensure that dimensions are easily readable and distinctly placed so that anyone reviewing the drawing can instantly grasp the specified measurements. Redundant dimensions can clutter the drawing, making it harder for anyone to understand the intended design and specifications.

In contrast, the other options represent established conventions in dimensioning. For instance, dimensions are typically read from the bottom right-hand side to maintain consistency and ease of interpretation. Using millimeters as a standard unit ensures clarity and commonality, particularly in engineering and manufacturing contexts. Additionally, having arrowheads that touch projection lines provides visual clarity on what is being measured, making it easier to relate dimensions to the specific features of the drawing.