A Primary Key is created by defaut as a Clustered Index, unless a Clustered Index already exists on the table, which is really to say that the Clustered Index does not have to necessarily be the Primary Key. It is the semantics of how you are phrasing the question that is a little tricky and confusing for some. However, as you state Paul in your explanation, the more important piece to emphasize here is the reasoning of where you decide to put your Clustered Index on your table. Application functionality can drive this at times and a PK is not always the first choice . That said, primary keys do tend to make make good candiates for the CI simply because they and their auto-incrementing data (singleton PK columns are many times IDENTITY columns as well, although not required to be) automatically satisfy two main requirements of a Primary Key. NO NULLS and Unique data, where Unique Keys can allow one NULL and are created Non-Clustered by default and can have more than one UK on a table I believe. Both PK's and UK's can be made up of more than one column as well. . 🙂