As the horizon, Aker was also seen as symbolic of the borders between each day, and so was originally depicted as a narrow strip of land (i.e. a horizon), with heads on either side, facing away from one another, a symbol of borders. These heads were usually those of Lions. Over time, the heads became full figures of lions (still facing away from each other), one representing the concept of yesterday (Sef in Egyptian), and the other the concept of tomorrow(Duau in Egyptian)
Since the horizon was where night became day, Aker was said to guard the entrance and exit to the underworld, opening them for the sun to pass through during the night. As the guard, it was said that the dead had to request Aker to open the underworld's gates, so that they might enter. Also, as all who had died had to pass Aker, it was said that Aker annulled the causes of death, such as extracting the poison from any snakes that had bitten the deceased, or from any Scorpions that had stung them.