Key decoders are designed to make locksmiths' jobs easier. These tools are produced to exact manufacturer specifications to quickly decode the cuts on any compatible cut key. A key code or bitting is a series of numbers representing the cuts on a key. Typically higher numbers represent deeper cuts while lower numbers represent shallower ones.
A task as simple as rekeying a lock is greatly simplified when you no longer have to guess which pins to use. This can become a particularly frustrating task when using universal pin kits--a frustration easily mitigated with a very inexpensive investment. Each step inside the tool corresponds with a cut of a particular depth. Inserting the key and seeing which step the cut bottoms out in determines the depth of the cut in that particular space. With a little practice a standard five cut key can be decoded in under 15 seconds. Another use for these tools is the creation of code cut, or "factory original," keys. We've all seen worn or poorly cut keys in desperate need of a makeover. Decoding these worn keys to factory specifications is a fairly simple task with these tools after which code cutting equipment of your choice can be used to make a fresh key.
It is however important to realize the limitations of these tools. Each gauge is specific for one set of depth information. This depth information is generally very specific from manufacturer to manufacturer, and even from series to series within one particular manufacturer (a concern especially for Corbin/Russwin and some other commercial manufacturers). In other words a number 3 depth in a Schlage key is going to be different from number 3 depth in a Kwikset key. With this in mind it is obviously important to use the proper key decoder for whatever type of key you are working with.