Sometimes you may find that your models are able to charge an enemy unit in the flank or rear. This is particularly good because an attack from an unexpected direction gives you an advantage in combat.
A charging unit's position when the charge is declared determines whether it charges into the front, flank or rear of the enemy unit. If the charging unit is in the target's frontal zone when the charge is declared then it charges into the front.
As units generally begin the game facing each other, this is the most common situation that will arise. However, if the charging unit comes from the flank zone, it charges into the side; if in the rear zone, it charges into the rear. Whether a charger is in the front, flank or rear of its intended target is determined before charges are declared, so make sure you check before charge reactions are declared.
If a charging unit straddles two zones, then the unit is considered to be in the zone where the majority of the models in its front rank are. If there is no clear majority, roll a dice.