The heavy line labeled with letters at both ends that shows the direction from which a sectional view is taken is called a ______ line

The term "cutting plane line" refers to the heavy line that, when labeled with letters at both ends, indicates the direction from which the sectional view is taken. This type of line is essential in technical drawings, as it provides a clear and specific representation of where the model is being 'cut' to create a sectional view. It helps in understanding the internal features of the part being illustrated.

In practice, the cutting plane line is often depicted using a thicker line style than other lines in a drawing, making it easily identifiable. The letters at the ends correspond to the view that will appear in the sectional drawing, facilitating communication regarding the orientation of the section being viewed.

The other options do not fulfill this specific role. Reference lines in drawings are used for indicating dimensions or points of interest; break lines denote sections of a drawing that are omitted for clarity; and visible lines define edges that are visible in the view but do not indicate a sectional cut. Each of these plays a vital role in technical drawings but does not represent the function of indicating a section view direction, which is why the term "cutting plane line" is accurately assigned to the correct answer.