In an ancient hymn to the sun god, it is said:
'Oh Amun-Ra, the gods have gone from thee. What flowed forth from thee became Shu,
and that which was emitted by thee became Tefnut;

thou didst create the
nine gods at the beginning of all things, and thou wast the Lion God
of the Twin Lion Gods.'

The Twin Lion Gods were Shu and Tefnut.
Collectively, they could be known as the Aker gods,
the lions of yesterday and today, each facing away from one another.

Tefnut represented the setting sun, while Shu represented the rising sun.
Shu can be depicted as a lion, or a lion-headed man,
but his usual form is that of a man wearing upon his head one,
two or four feathers.
Aker was also seen as symbolic of the borders between each day

. Since the sun reaches its peak (its solstice) in the zodiac of Leo,
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 today (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.

As the Egyptians believed that the gates of the morning and evening were guarded
by Aker, they sometimes placed twin statues of lions at the doors of their
palaces and tombs.