A lapel knife is a small, easily concealed blade often used by spies in WWII. It can be hidden in a lapel, as its name suggests, or it can be dropped into a boot, slid into a sock, or even hidden in a tie.
My lapel knife is made from a high carbon steel circular saw blade. I cut it out with an angle grinder and shaped/sharpened it with a bench grinder. The blade is chisel ground, meaning it is sharpened on one side of the blade only, like a chisel. LOA is ~ 6.5cm, with 5 cm for the blade.
I'm not sure how the knife was originally intended to be held, but I hold it between my thumb, middle and index fingers, almost like it's a key. I DO NOT recommend holding it between thumb and the middle of the index finger. I tried it this way and got a nasty cut almost immediately.
Now, this is not a knife one would use to stab through a door. It's for quick, jabbing strikes to the face, eyes, and throat. Maybe some thug's trying to pull you from your car, and you give him one heck of a poke on the back of the hand. He's going to want to let go pretty darn quick. Also, as I mentioned in my Kakute ring 'ible, head wounds bleed profusely. If this knife opens a 6" gash in an attacker's forehead, the blood streaming into his eyes prevents him from seeing clearly, giving you time to kick is legs out from under him and get away.
DISCLAIMER: I AM NOT RESPONSIBLE FOR ANY DAMAGES INCURRED, INTENTIONAL OR OTHERWISE. MAKE AND CARRY AT YOUR OWN RISK.