What’s the difference between a bolt and a screw? Bolts hold the material in place, but the material holds the screw in place. Bolts use some kind of nut or threaded interface to stay in place, while screws are held in place by the materials they are used in. Bolts are used in applications as varied as railway trestles or to attach the wood frames of buildings to their concrete foundations.

When considering the right bolt for any given application, there are several important factors to consider. The first is bolt type (carriage, lag, elevator, etc.). For example, a carriage bolt has a square neck that keeps it from turning once it’s fastened, while a lag bolt has threads designed to bite into the surface and resist pull-out.

Bolts come in a variety of materials, from plastics such as polyurethane to metals such as titanium and stainless steel. In general, plastic is more corrosion resistant than metal, is nonmagnetic and does not conduct electricity. Metal has higher tensile strength and resists heat better than most plastics.

Another consideration is grade for standard bolts and property class for metric bolts. These two characteristics simply indicate the bolt’s level of strength.

Bolts don’t have as many head styles as screws, but you can choose between a countersunk bolt such as an oval head or one with low head clearance such as a large diameter round. Bolt heads also come in hex, round, square, and truss styles. Countersunk bolts are measured from the top of the head to the tip, while most bolts that have their head above the object’s surface are measured from the bottom of the head to the tip. Shoulder bolts are measured by the length of the shoulder, which is the total length minus the head height, and the length of the threaded portion of the bolt.

The bolt size name indicates the external diameter of the threaded portion of the bolt, followed by the threads per inch for standard bolts and millimeters per thread for metric bolts. Standard bolts are measured in inches and metric bolts are measured in millimeters.