The Deadly Force Triangle consists of three determining criteria: capability, opportunity, and intent. To justify the use of deadly force against another human being, all three criteria must be met. 

Capability is the means by which an individual may cause harm to you or others. The capability could be a weapon such as a knife, firearm, or baseball bat; in some situations it could be simply the use of fists and feet. A large man being threatened by a smaller attacker using fists may not be justified in using deadly force, but a smaller woman is likely justified in using deadly force against a larger attacker using fists.

Opportunity is the effective range at which the capability can be implemented to cause harm. An individual with a rifle has the opportunity to inflict harm from hundreds of yards away, while an individual with a knife must be within several feet to do the same. 

Intent is the motivation of an individual to inflict harm, and is the hardest to determine. A potential threat could walk through a parking lot with a baseball bat and come within striking distance of you, but that same individual could simply be walking home from batting practice. Determining intent requires being proactive; if you change your position and the potential threat follows, this can determine intent. McCall Tactical teaches the Deadly Force Triangle so it becomes instinctive. In a critical incident, there is never time to think through all the possibilities; instead, the decision will occur instinctively.