If you have assembled IKEA furniture, then you have used an Allen key. These slim L-shaped devices are usually the only tools included in the package, even if you need someone else to do the job. There is a good reason for this: most people don't have an Allen key, and when you need it, there is no other way.

For some reason, allen wrenches (also called hex wrenches, hex keys, or hex wrenches) are not included in many basic kits, although they are cheap and take up almost zero space. Their hex heads (hence the name "hex") are used to loosen and tighten bolts and screws with hex sockets. They can be found in everything from bicycles to furniture to certain types of electronic products. If you don't have something to hold together with Allen screws, it is only a matter of time before you have it-this time it may not be equipped with its own Allen key. In other words, you should definitely buy it yourself. Here is how to choose a set of wrenches.

Allen wrenches are sold in sets, and are designed in various sizes to suit different sockets. You can buy metric or imperial wrenches, depending on which type of screw or bolt you are using. Most sets are available in sizes of 1-10 mm.

If you only buy a set of wrenches, please set it to metric. Most manufacturers in the world use the metric system, so the metric wrench is a must for basic maintenance. But if you live in the United States, it doesn’t hurt to have an imperial set, because this is the system used by most screws and bolts sold in hardware stores in the United States. Imperial and metric wrenches are actually not interchangeable, so it is wise to have one set of each, not superfluous-besides, they won't take up much space in the toolbox.

Once you decide to buy metric or imperial wrenches (or each set of wrenches), you need to further narrow your choices by shape. Some allen wrenches have flat ends, while others have a "ball head", which is almost what it sounds like. The work of the two types is the same, but the ball-end wrench is more versatile, because the round shape is easier to operate in a small space. You can use the hard-to-reach screws from the ball end and then switch to the flat end to get the job done.

Even if you never need an Allen wrench that is not a package deal, you will almost certainly need it at some point. Hexagon socket screws are everywhere-when one of them deteriorates, you will be happy to have your own set of wrenches.

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