If you believe in self-preparedness you absolutely must carry a knife. Even if you don’t carry a gun you must carry a knife. Even if it is not a proper knife, carry a multi tool with a cutting blade. You are not equipped to face the dangers of this world if you have no cutting tool on your person. I got stuck in a seat belt in an old car that would not release when I was 12 years old. That was the last day I ever left the house without a knife on my person. The only time I don’t have a knife on me is when I have to get on an airplane. Otherwise, I have a knife, or at least a small multi tool with a cutting edge if I need to be in a restricted environment. The ability to literally cut yourself free from something can be a matter of life or death.
As it pertains to self-defense, the knife is secondary for me. The handgun is my primary defensive tool. With that said, the knife is an important force tool as well and it is integral to a well-rounded defensive plan even if, like me, you are not a knife guy. Obviously, there are some places you may have to go that deny you your civil liberties and you can’t carry a gun but perhaps you can carry a knife. In that case it serves as a primary weapon that is a hell of a lot better than having only your hands and teeth. But, even when armed with your handgun, the knife serves a potentially important role in your self-defense.
For the individual that always carries a gun the knife may come into play in the most unfortunate event of a retention or weapons access issue. If you are in a contact distance struggle, perhaps on the ground, the knife is often more deploy-able than the gun, depending on how you carry it. My advice is to carry your knife accessible to your support hand. Most gun guys clip a folding knife to the pocket of the strong hand. This does you no good if your strong hand is tied up in a contact distance struggle. Put that folder on the support side.
Deploying a folder under stress, especially with the support hand, is not easy. Sorry sunshine, you are going to have to practice if you want to be able to do it when the chips are down! If you carry a fixed blade keep it accessible to the support hand as well. If carrying a fixed blade center line it can be accessed with either hand, which is a huge benefit of fixed blades worn in the appendix position: I would submit that fixed blades are ideal if you are serious about the knife as a defensive weapon.
So, what knife do I carry? I have a few, but my favorite, believe it or not, is an Ontario Rat-1 folder. The crazy thing, you can buy these suckers for less than 30 dollars on Amazon. When it comes to the gear I carry I am willing to spend money, but I carry the Rat because it is my favorite knife. It is not assisted yet it is the fastest deploying folder I have used. It is also indestructible. The steel is decent, AUS-8, but not great. You will need to sharpen it often. But, for my purposes it is a great knife and easily worth twice the price. I own several of them. Being so affordable it is easily replaced if lost, another benefit.
As stated, for an individual who is serious about the role of the knife in a defensive capacity going with a fixed blade is the obvious choice. There is, however, two issues with fixed blade knives: First, folders are more convenient. Clipping a folder on the pocket is generally more convenient than wearing a fixed blade in a sheath on the belt. Second, knife laws throughout the United States are pathetic and absurd. Even in many Shall Issue carry states, in which you can carry any handgun you want concealed as a permit holder, you can’t carry a concealed fixed blade knife legally. Folding knives are generally far less restricted.
For myself, a robust folder is my knife of choice due to less legal restrictions as well as sheer convenience. The knife is primarily an emergency cutting tool for me, although I do train with it to be able to deploy the folder quickly with a single hand if needed. Knives are completely personal preference, so carry what you like, but be sure you always carry it; your knife may be the tool needed to save yourself or someone else.