Metric, SAE, and Whitworth: While SAE, or standard-size sockets measured in inches, are widely used in older family vehicles, most modern vehicles (imported and domestic) primarily use metric hardware measured in millimeters. Choose a metric or SAE kit based on your needs, or keep both in your garage to ensure you always have the right tools for the job.

Wyeth sockets are a third type used almost exclusively on British vehicles. Whitworth nominal dimensions are in inches like SAE but reflect the size of the bolt shaft or the inside diameter of the nut. Therefore, a 5/8" Whitworth receptacle is larger than a 5/8" SAE receptacle. If you work on classic British cars like an MG, Rover or Jaguar, you'll probably need Whitworth hand tools. If not, stick with SAE and/or metric sizes.

Drive Sizes: Both metric and SAE sockets are available in three drive sizes: 1/4-inch, 3/8-inch, and 1/2-inch. Each requires a different ratchet and other accessories, such as extensions. Smaller sockets typically use smaller driver sizes, while larger sockets use larger driver sizes. If your socket set is limited to one driver size, choose 3/8-inch drivers to cover the majority of sockets you will use. However, the minimum and maximum drive sizes are also very useful. A small 1/4-inch drive socket can sometimes make it easier to reach small nuts or bolts in tight spots. The large 1/2-inch drive socket can handle more torque and is ideal for large fasteners that require a longer ratchet or breaker bar to loosen or fully tighten.

Key Points: Look down at the business end of the socket, the part that slides onto the nut or bolt head. What shapes do you see? Is it a six-pointed hexagon or a twelve-pointed "star"? Six-point sockets are sturdier than 12-point sockets because their inner walls are flush with all six sides of the hardware. The advantage of the 12-point socket is convenience; it slides into any of 12 positions on the hardware, making it easier to line up. While 12-point sockets are suitable for most light jobs, for larger hardware or applications requiring high torque, choose a six-point socket. In this case, the six-point sleeve is less likely to slip.

Shallow Sockets vs. Deep Sockets: Imagine that when fully tightened, the nut is one inch below the top of the bolt or stud it is connected to. A normal shallow socket may not even contact the nut until the top of the stud or bolt hits the end of the socket. In this case, a deep sleeve is needed to accommodate the long end of the bolt and fit tightly around the nut. Having some deep sockets on hand can be helpful in this situation. Without them, you may need a wrench instead of a socket to loosen the nut.

A spark plug socket is a special deep socket that every DIYer should have. These are deep enough to reach the base of the spark plug in place, and the spark plug socket contains a small rubber tab inside to protect the top of the spark plug from damage.

Impact sockets: Not all sockets are designed for use with power tools. If you use an air or electric impact wrench, you'll need a set of impact sockets designed specifically for use with these high-torque tools. Stronger construction allows the impact socket to withstand the forces of power tool use.

Some socket sets include a ratchet and accessories such as an extension or snap-off lever. If your tool box doesn’t already come with a ratchet and extension, it may be more economical to purchase a socket set that includes these key components. I often use extensions on sockets, so I usually always leave a medium-sized extension on each ratchet. If I can easily reach a piece of hardware without it (and I need enough force), I'll remove the extension and attach the socket directly to the ratchet.