To which circle is the center mark applied in a drawing for clarity?

The center mark is typically applied to the outer circle in a drawing to enhance clarity and to indicate the axis of rotation or symmetry for circular features. When drafting designs, it's important to provide clear visual cues, especially for parts that will undergo machining or assembly. The outer circle is usually seen as the boundary of the circular feature, and placing the center mark here helps users easily identify the center point around which other features may be oriented.

In the context of engineering drawings, the center mark serves as a reference point for other dimensions and can simplify the understanding of how different components relate to each other regarding their rotational axes. In contrast, applying a center mark to an inner or middle circle may not convey the same clarity of reference, as these circles often serve different purposes or may not be the primary focus for locating features on the part.