Use of double pointer in linux kernel Hash list implementation

Hi,
  I am trying to understand Linux Kernel implementation of linked list and
hash table. A link to the implementation is
here<http://lxr.linux.no/linux+v2.6.34/include/linux/list.h>.
I understood the linked list implementation. But i am little confused of why
double pointers is being used in hlist (**pprev). Link for hlist is
here<http://lxr.linux.no/linux+v2.6.34/include/linux/list.h#L554>.
I understand that hlist is used in implementation of hash table since head
of the list requires only one pointer and it saves space. Why cant it be
done using single pointer (just *prev like the linked list)? Please help me.

Thanks,
Bala