How different is the phonology and morphology of nontemplatic (concatenative) word formation from that of templatic (nonconcatenative) word formation ? We will focus on the Arabic verbal system, the prototypical example of templatic morphology, with the aim of deriving some of its distinctly special traits from basic principles. The key novel aspect of the approach is its focus on paradigms. The main result is that the paradigm coupled with general phonotactic constraints sets limits on the theoretically possible diversity of stems within that paradigm. The core analysis will be on Classical Arabic. However, we will bring in data from dialects which justify the approach and/or permit further theory development.