Technically, these are called "weapon carrying conditions", but I didn't want people to think this was an article about "under which circumstances" you would carry a firearm.

Not all firearms are capable of being in all of these conditions.  For example, if your firearm does not have a mechanical safety lever, the gun can be in Condition Zero, but not Condition One.

WEAPON CARRYING CONDITIONS

• Condition Zero:  Magazine inserted, round in the chamber, bolt is forward, ejection port cover is closed, hammer is cocked, and the weapon is not on safe (or incapable).
• Condition One:  Magazine inserted, round in the chamber, bolt is forward, ejection port cover is closed, and the weapon is on safe.
• Condition Two:  Magazine inserted, round in the chamber, bolt is forward, ejection port cover is closed, hammer uncocked (forward), and the weapon is on safe.
• Condition Three:  Magazine inserted, chamber is empty, bolt forward, ejection port cover is closed, hammer uncocked (forward), and the weapon is on safe.
• Condition Four:  Magazine removed, chamber empty, bolt forward, ejection port cover closed, hammer uncocked (forward), and the weapon is on safe.