We must now examine the conduct of Officer McFadden in this case to determine whether his search and seizure of petitioner were reasonable, both at their inception [p28] and as conducted. He had observed Terry, together with Chilton and another man, acting in a manner he took to be preface to a "stick-up." We think, on the facts and circumstances Officer McFadden detailed before the trial judge, a reasonably prudent man would have been warranted in believing petitioner was armed, and thus presented a threat to the officer's safety while he was investigating his suspicious behavior. The actions of Terry and Chilton were consistent with McFadden's hypothesis that these men were contemplating a daylight robbery -- which, it is reasonable to assume, would be likely to involve the use of weapons -- and nothing in their conduct from the time he first noticed them until the time he confronted them and identified himself as a police officer gave him sufficient reason to negate that hypothesis. Although the trio had departed the original scene, there was nothing to indicate abandonment of an intent to commit a robbery at some point. Thus, when Officer McFadden approached the three men gathered before the display window at Zucker's store, he had observed enough to make it quite reasonable to fear that they were armed, and nothing in their response to his hailing them, identifying himself as a police officer, and asking their names served to dispel that reasonable belief. We cannot say his decision at that point to seize Terry and pat his clothing for weapons was the product of a volatile or inventive imagination, or was undertaken simply as an act of harassment; the record evidences the tempered act of a policeman who, in the course of an investigation, had to make a quick decision as to how to protect himself and others from possible danger, and took limited steps to do so.