Sometimes, until you actually go looking for a specific item, you have no idea how many variations there are. Knives are an item that falls into that category. There are thousands of types of knives when you look for them, and picking out survival knives is more complicated now than ever before.
Three of the main types of knives are automatic, spring assisted and butterfly. Automatic knives open with the touch of a button and can open from the front or side. Many states have laws that pertain to owning this kind of knife, so although very fun to own, make sure you know your local laws before buying one.
Also, the blade should be between four to six inches long because anything bigger than that is often very hard to control. The blade thickness should be between 5/32 and 8/32 of an inch. Any blade that is thinner than that is too flexible to accomplish much and blades thicker than that lack the finesse that survival knives are often called upon for.