3-leg perimeter: A "3-leg perimeter" in the context of security or defense refers to a tripartite defensive boundary set up around a specific area or object. This type of perimeter typically involves three distinct zones or segments, each serving as a layer of defense or security. The concept is based on the principle of depth in security, where multiple layers provide enhanced protection. Each "leg" or section of the perimeter might consist of different security measures, such as physical barriers, surveillance systems, or patrols, and is designed to detect, deter, or delay potential threats. The use of a 3-leg perimeter is common in military tactics, high-security installations, and other scenarios where robust defense mechanisms are essential.

History: In military history, the use of layered defenses can be traced back to ancient and medieval fortifications. Castles, for example, often had multiple layers of walls, moats, and guarded gates to protect against invaders. The idea was that even if one layer was breached, the subsequent layers would continue to offer resistance and delay the attacker, providing the defenders with more time to react.

In modern security, this concept has evolved into what's known as "defense in depth." This strategy involves multiple layers of security controls and measures placed throughout an information system. The layers include physical, technical, and administrative controls. The purpose is to provide redundancy in the event a security control fails or a vulnerability is exploited.