And to get even more detailed -- not all guns have a decocker. 1911 and clones come right to mind. CZ-75 and clones as well. This lets you carry the gun cocked but with safety on (since the decocker (always? I know of no exceptions but I am far from expert) is part of the safety, and actuated when the safety goes on). Decocking with these guns is done by manually lowering the hammer, carefully.
And, at least one decocker is potentially unsafe to use (50s era Commie milsurp CZ-52).
Oh, yeah, and lets tell him about the Daewoo system, too, just to confuse things even more.