I have been asked a couple times recently what the "Conditions of Readiness" are for pistols when carrying them. This specifically was designed for the 1911, but it can be translated over to other guns by using some logical reasoning.
Condition 0: Round in the chamber, hammer cocked, safety Off
Condition 1: Round in the chamber, hammer cocked, safety On
Condition 2: Round in the chamber, hammer uncocked
Condition 3: Empty chamber, hammer uncocked, loaded magazine in weapon
Condition 4: Empty chamber, no magazine in weapon
For the 1911, as well as DA/SA guns like HKs and older S&W autos, this all makes sense. For a DAO striker-fired pistol like a Glock or M&P, it's a bit different. There is some debate on whether a loaded Glock or M&P is considered C-0 or C-1. Some have contended that if you have your finger on the trigger, it's C-0, but if the finger is indexed, it's C-1 since they have internal safeties. Thing is, these conditions refer to conditions for CARRYING the gun, not handling it.
For Glocks, XDs, M&Ps, Kahrs, etc.
C-0: Round in the Chamber with loaded magazine
C-1: APPLICABLE TO M&P's w/ THUMB SAFETY ONLY
C-3: Loaded mag, empty chamber, trigger pulled to disengage striker
For carbines and battle rifles (slight variation):
C-0: Round in the chamber, loaded magazine, safety selector on Fire/Semi/Auto
C-1: Round in the chamber, loaded magazine, safety selector on safe
C-3: Chamber empty, loaded magazine, weapon on SAFE (this varies b/c you don't pull the trigger on rifles since you need to engage the safety)
C-4: unloaded and on SAFE