Airframe components are held together with a variety of threaded metal fasteners. These are grouped into the familiar categories of bolts, nuts, and screws. However, even though these simple devices may look the same as their hardware store counterparts, aircraft grade fasteners are manufactured to much higher standards of strength and durability. For example, a typical bolt purchased from a hardware store is made of low carbon steel with a low tensile strength (usually in the range of 50,000 to 60,000 psi). It can bend easily and has little resistance to corrosion. Aircraft bolts, on the other hand, are made from corrosion resistant steel that has been heat treated to achieve tensile strengths above 125,000 psi. They also have more contact area in the threads which transmits the load better and further increases strength. This can be checked by means of a "nut wobble test.” When a nut is screwed onto a typical hardware store bolt, it will wobble slightly if you rock it back and forth with the tips of your fingers. That won’t happen with an aircraft grade bolt and nut. The threads also have a unique feature called a "J” thread which gives them a distinct radius at the bottom of the grooves that reduces stress risers and increases fatigue resistance by as much as 40 percent. Hardware store bolts are usually coated with chromate, whereas aircraft grade bolts are plated with cadmium, which gives them much greater resistance to corrosion.

Prior to WWII, each branch of the US military had developed its own standards for aircraft hardware. Eventually these standards were consolidated under the designations AN (Air Force-Navy), MS (Military Standard), and NAS (National Aerospace Standard). In addition to these labels, aircraft hardware pieces have numbers, letters, and other markings to identify their size, type of material, and other characteristics. For example, a bolt with the markings AN4-4A would have the following specifications:

AN — manufactured according to Air Force-Navy standards

4 — the diameter of the bolt shank (the grip and the threads) in 1/16 inch increments

4 — the length of the shank in 1/8 inch increments

A — signifies that the shank is un-drilled (no letter here signifies a drilled shank)