You will no doubt hear the players working to position their characters during combat to get advantage on their attacks. The Dungeon Masters Guide contains optional rules for giving advantage when flanking and attacking from behind. However during the play test, we devised our own rule. To simplify things, we simply gave the attacker advantage if they were attacking from the square directly behind the defender. We found if we allowed the attackers in all the rear facing squares to have advantage, this tended to tilt the fight too much towards the side with multiple attackers (usually the party), while giving advantage only to the attacker directly behind made for a much more even scenario while giving a bonus to the attacker that could capitalize on the tactical situation (usually the monks).