Below you can see two photos of an adjustable base anchor sling with a prussic. The photo on the left shows the spliced eye of the dead eye sling fed through the ring of the prussic. The photo on the right shows the prussic with the ring fed through the spliced eye of the dead eye sling.
Now, what is the difference?
The setup in the left photo needs a stopper knot behind the prussic to be a safe setup. The stopper knot is not a secondary safety but a necessity and needs to be placed as close as possible to the prussic. The tree works like a hand on the prussic, compressing the prussic and therefore allowing the prussic to open. The dead eye sling can creep loose.
In the right photo the spliced eye of the dead eye sling holds the prussic in place. When the climber loads the anchor the prussic is pulled away from the tree trunk without being compressed, choking the sling tighter to the tree base. It is good practice to always back up a prussic with a stopper knot where the prussic is not under constant control.
We at Pro Climb highly recommend using the setup as shown in the right photo.