sometimes we want to go from to bbut at the same time from to in that casewe add previous pointer in addition to the next pointeras you can see from the figurewe let both the last and the first nodes point to noneto indicate that we have reached they form the boundary of our list end-point the first node' previous pointer points to none since it has no predecessorjust as the last item' next pointer points to none because it no successor node you might also be creating tiles for tile-based game in such caseinstead of previous and nextyou might use northsoutheastand west there are more types of pointersbut the principle is the same tiles at the end of the map will point to none