The image below is a set of alternating balusters crafted for a customer a year or so back. The profile of the balusters is the same except for the center part.
The twist on these is called a barley twist as opposed to rope twist in the post below.
These are 1 3/4″ at the squares with a 3/4″ pin on top.
Below is a finished project with square top balusters. Thanks Jess for the final pictures – Anera Group
These 3inch wood balusters were for a customer whose wife saw them, or something similar, in a Pottery Barn catalog. He asked me to reproduce them from the catalog image. They alternate from one profile to the next. They were crafted from poplar and were painted.