Center lines show the axis of a hole or a cylinder. Center lines start and end with long segments with a short dash in the middle. They should extend beyond the edge of the feature they are placed on a short, but distinct amount.
They can be used to show the position of more than one hole or cylindrical element by extending from object to object. If they are very small and it is not confusing, they do not have to be broken.
Center lines should not start or stop where the end lines up with another line, and they should not extend from one view to another.