It is possible to have a linked list in which the last link points back to the front of the list rather than to null or nil. This is called a circularly linked list.
This may be convenient for data such as a list of vertices of a polygon, since it makes the data uniform: each vertex is followed by its successor.
Circularly linked lists must be used with care, since they make it easy to write a program that gets into an infinite loop.
A doubly linked list has link pointers that point both forward and backward. This makes it easy to insert or remove items at both ends.