The threaded end of bolt usually has a nut screwed onto it to complete the assembly. The threaded end of a screw may fit into a female receptacle, or it may fit directly into the material being secured. A bolt has a fairly short threaded section and a comparatively long grip length or unthreaded portion; whereas a screw has a longer threaded section and may have no clearly defined grip length.A bolt assembly is generally tightened by turning the nut on the bolt: the head of the bolt may or may not be designed for turning. A screw is always tightened by turning its head.