I've written about alternating balusters in a few different places. And so I thought I would post a picture of this particular stairway in Massachusetts with alternating balusters. We turned the balusters as well as the newel for this one. The 8 inch newel is what I call a pottery newel because it was first conceived from a image in a Pottery Barn Catalog.

As you can see there are three balusters per tread.  The barley twisted baluster is flanked by a plain tapered baluster.  Both the barley twist balusters and plain baluster, however, have the same base profile.  Secondly note that the tops follow the "rake angle of the handrail.  The balusters are poplar as well as the newel.