You can select the silhouette edge of a rounded surface, such as a cylinder.
To set silhouette edge visibility, click
Options 
(Standard toolbar) or and select
Dynamic highlight from graphics view.
Silhouette edges are included in box-select operations.
To select a silhouette edge:
- Open a sketch on a model face, plane, or in an active drawing view.
- Select one of the tools:
- Convert Entities
- Offset Entities
- Smart Dimensions
- Add Relations
- Move the pointer over a cylindrical face until the pointer changes to
.
- Select the highlighted edge.