Directional drilling can involve both the initial pilot hole and enlarging the hole by the reaming technique.

A pilot hole is typically small diameter that is first drilled during horizontal directional drilling (HDD) before the hole is enlarged. A pilot hole is drilled from the entrance pit to the exit pit and is directionally controlled to cover the desired route.

Reaming is the process of enlarging an existing hole and is a regular part of any underground drilling project. A hole can be reamed either in a forward or backward direction.