What is a bitting and why you should hide it.
The bitting refers to the number and depth of the cuts on a key. The higher the number, the deeper the cut. Most original keys will have a bitting stamped on them (see below). This is great for the locksmith when he wants to cut a new key by code (meaning not copy it, but cut each bitting.) This is not so great if you have an original key on your key ring and a thief copies down the bitting or commits it to memory. Once they have that, it's just a matter of cutting a new key by code. So, what's the solution?
1. Never have an original key on your key ring. I secure all my original keys in my safe.
2. Have a copy made and use that. It won't have a bitting on it.
3. When you make a copy of a key, always use the original to do it. The old adage of a copy of a copy of a copy is true.
So, looking at the key below, we see that the bitting is 33688. Look at the key and you can see that the first two cuts (starting from left to right) are 33. Fairly shallow cuts, then 6, a little deeper, then 88 even deeper than 6. So the bitting does match the cuts in the key.
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I had no idea! 
Of course, I have a key machine at home, so I don't have to interact with the folks at the big box stores for that. But I do have to interact with some of my neighbors who say, "Hey can you cut a few keys for me?, or "This key I had made seems to be sticking, can you look at it?"
"Use some Tri-Flow." 
