The particularity of the plain stringer staircase is in that  the spindles rest on the edge of the stringer, As can be seen on the photograph bellow, unlike cut-stringer staircases where they rest on the top of the tread. 

 

This type of staircase is commonly known in France as the "French staircase" and, because it is slightly "heavier" in appearance, it is often better suited for "farmhouse" types of properties.