to simplify programmingit is convenient to add special nodes at both ends of doubly linked lista header node just before the head of the listand trailer node just after the tail of the list these "dummyor sentinel nodes do not store any elements the header has valid next reference but null prev referencewhile the trailer has valid prev reference but null next reference doubly linked list with these sentinels is shown in figure note that linked list object would simply need to store references to these two sentinels and size counter that keeps track of the number of elements (not counting sentinelsin the list figure doubly linked list with sentinelsheader and trailermarking the ends of the list an empty list would have these sentinels pointing to each other we do not show the null prev pointer for the