Two half-hitches
Two half-hitches is a type of knot, specifically a binding knot or hitch knot. One variety consists of an overhand knot tied around a post, followed by a half-hitch. This knot is less often referred to as a clove hitch over itself, double half-hitch, or full-hitch. The following three-step process for tying the two half-hitches is also explained in the image gallery below. Click on the images for high-resolution versions. To release the knot, pry apart the two hitches with a bending motion. However, it can often be difficult to  untie. To help avoid this problem, tie a slipped variation: in the second half-hitch, pass through a bight, as when tying your shoe, rather than the entire free end. Ashley Lesson #1781 has quotations on using two half-hitches for safety: