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Cover Effective Cover

A team covers each other by about watching each others backs, and making sure you can have a withering crossfire on opponents. It is generally better if your team is to stick near each other, to have people looking in every possible direction that an opponent could be covering from.  So your total lines of visions protect you from surprise attacks.  Simply having the entire team not facing the same direction is often a step in the right direction.  

Also its better to have your team to be in decently close proximity to each other, so whenever some one has some spare time, they can take pot shots at other opponents backs.  It is generally better then to surround your opponents, rather then to be surrounded.  Being tightly grouped together increases the damage bombs to do, and severely restricts mobility. 

Good luck.