A bolt is a cylindrical fastener with a head on one end and external threads along its shaft. A nut is the small, typically hexagonal block with internal threads that screws onto the bolt. Together, they form a pair: the bolt passes through the materials being joined, and the nut tightens onto the exposed end to clamp everything in place. They’re two halves of one fastening system, and neither works without the other (or without something playing the other’s role).
How Each Part Works
The bolt is sometimes called the “male” fastener. Its threads are cut along the outside of a cylindrical shaft, spiraling from the tip toward the head. The head, usually a hexagonal shape, serves two purposes: it provides a surface for a wrench to grip, and it acts as a stop that prevents the bolt from pulling all the way through the material.
The nut is the “female” counterpart. Its threads are cut on the inside of a hole running through its center. These internal threads are sized to match a specific bolt, so when you turn the nut onto the bolt’s shaft, the two thread patterns interlock. As you keep turning, the nut travels along the bolt and pulls the joint tight.
This threading relationship is the core difference. External threads on the bolt, internal threads on the nut. Every other distinction flows from that basic split in roles.
How They Create a Secure Joint
When you tighten a nut onto a bolt, you’re not just screwing two pieces of metal together. You’re stretching the bolt very slightly, like pulling on a stiff rubber band. That stretch creates tension in the bolt’s shaft, and that tension is what generates clamping force, the pressure squeezing the joined materials between the bolt head on one side and the nut on the other.
The amount of clamping force depends on how much torque you apply when tightening. This is why torque wrenches exist: in structural, automotive, and industrial work, bolts need to be tightened to a specific value. Too little torque and the joint can loosen over time. Too much and you risk snapping the bolt or stripping the threads.
Bolts Don’t Always Need a Nut
In many assemblies, the bolt threads directly into a hole that’s been pre-cut with matching internal threads (called a tapped hole) rather than using a separate nut. Engine blocks, machinery housings, and metal frames often use this approach. The tapped hole essentially plays the nut’s role, providing the internal threads that grip the bolt.
A nut-and-bolt assembly is more common when you’re joining materials that are too thin, too soft, or too awkward to tap threads into. You drill a clearance hole (just wide enough for the bolt shaft to pass through, but too small for the head), slide the bolt through, and thread a nut onto the other side. This is the setup you’ll see in wood decks, furniture, fencing, and most DIY projects.
Common Types of Bolts
Not all bolts look the same, and the differences matter for specific jobs.
- Hex bolts are the standard. They have a six-sided head and work well for metal-to-metal connections where you can access both sides of the joint with a wrench.
- Carriage bolts have a smooth, dome-shaped head with a small square section just below it. The square neck digs into wood and prevents the bolt from spinning when you tighten the nut, so you only need a wrench on one side. They’re widely used for fences, decks, gates, and boat building.
- Flange bolts have a built-in washer-like flange under the head that spreads the load over a wider area, reducing the chance of the bolt pulling through thinner materials.
Common Types of Nuts
Nuts also come in variations designed for different situations.
- Hex nuts are the standard six-sided nut and the one most people picture. They pair with hex bolts for general-purpose fastening.
- Lock nuts (or locknuts) include a feature that resists loosening from vibration. Some have a nylon insert that grips the bolt threads; others have a slightly deformed shape that creates friction.
- Wing nuts have two flat “wings” you can grip with your fingers, allowing hand-tightening without tools. They’re useful for joints that need to be assembled and disassembled frequently.
- Flange nuts have an integrated washer base, similar to flange bolts, which distributes pressure and can eliminate the need for a separate washer.
Tools for Each
Because nuts and bolts have different shapes and sit on opposite sides of a joint, you often need two tools at once: one to hold the bolt head and another to turn the nut (or vice versa). Open-end wrenches and box-end wrenches are the most common choices. An open-end wrench slips onto the fastener from the side, while a box-end wrench fits completely around it for a more secure grip. Adjustable wrenches can handle multiple sizes by sliding one jaw wider or narrower.
For precision work, a torque wrench lets you tighten to an exact specification. Socket sets are also popular because they combine a secure fit with speed, especially in tight spaces where a standard wrench can’t make a full turn.
Bolt vs. Screw: A Common Mix-Up
People often use “bolt” and “screw” interchangeably, but they serve different roles. A bolt is designed to pass through a clearance hole and be secured by a nut (or threaded into a tapped hole). A screw is designed to thread directly into the material itself, cutting or forming its own threads as it goes. Wood screws, drywall screws, and sheet metal screws all work this way. If you’re using a nut on the back end, you’re working with a bolt. If the fastener grips the material directly with its own threads, it’s a screw.