Shoulder screws are fasteners with an unthreaded shank. The unthreaded shank is long when compared to its threaded portion, which is located under the head. Shoulder screws can be used to attach one machine part to another, or as a plain bearing in places where rotary or sliding motion is required. Shoulder screws can be made from steel or stainless steel, and are often heat-treated with a black oxide finish. There are two basic head styles: hex and slotted. Shoulder screw dimensions are measured in millimeters (mm) or inches (in), and carry lettered designations such as A (nominal), B (shoulder), C (diameter), D (head length), E (thread size), and F (thread length). Key size, weight, and thread size are important parameters to consider when selecting shoulder screws.