A crime of opportunity can't happen if you take away the opportunity.
The Rock Lock's heavy-duty hinged locking shackle fits practically all standard 6-string acoustic and electric guitar necks. It measures approximately 2-1/8" x 7/8" inside (54.0mm x 22.3mm). It's not designed to fit classical or 12-string guitars, or basses. The entire system fits in the string compartment of most guitar cases, so you'll always have it with you.
This is the updated Rock Lock version 2. Two keys are provided, along with a special key code for securely ordering replacements from the manufacturer if you lose the originals.
Common sense about security: The 40" (102cm) Rock Lock cable is vinyl-covered tough braided stainless steel. It's made to withstand most hand wire-cutting tools, which most people obviously don't carry around. Let's face it, if a thief comes to your gig armed with serious tools, he can defeat most any security device. The Rock Lock does a good job of thwarting everybody else. And isn't that a whole lot better than no security at all?