What is the main feature of external threads?

External threads are characterized by forming a uniform, spiraling groove on the exterior of a cylindrical object, such as a bolt or screw. This spiraling groove allows for effective engagement with corresponding internal threads, facilitating secure fastening and providing a means for mechanical connection. The design of external threads is critical for ensuring that they can successfully interlock with internal threads found in nuts, taps, and other components, thus making them essential in various applications, including machinery and construction.

The other options do not accurately describe external threads. For instance, external threads are not located inside a hole, as that would refer to internal threads. Similarly, smooth and rounded surfaces are indicative of a non-threaded finish rather than the defining characteristics of threads, which are meant to provide grip. Finally, while external threads do play a crucial role in fastening applications, they are not exclusively used for that purpose, as they can also be involved in the function of adjustments and components that need to be assembled or disassembled.