Where should an extension line begin in relation to notes in a drawing?

The correct choice is that an extension line should begin at the vertical center of the note in a drawing. This practice ensures that the extension lines are clearly associated with the notes they are intended to reference, providing clarity and precision in technical drawings. By starting the extension line at the vertical center, it helps to maintain a consistent visual relationship between the dimension and the geometric feature it describes, which is critical for accurate interpretation and execution of the design.

This placement also assists in avoiding confusion that may arise from extension lines that start at arbitrary spacing or from different points. It helps in establishing a standardized method for dimensioning, which is fundamental in technical drawing conventions. Adhering to this uniformity aids different stakeholders, such as designers, engineers, and builders, in accurately understanding measurements and specifications, thus enhancing communication and reducing errors in construction or manufacturing processes.