There are three different doweling guides for the Jessem doweling jig (model # 08350) that you can purchase separately or is included in the master kit (not affiliated).
See: What Is a Jessem Doweling Jig? and Dowel Size and the Rule of Thirds and Half
The Jessem 3/8 Inch Doweling Guide:
The 3/8 inch doweling guide is generally the most used guide size because most woodworking projects use 3/4 inch to 1″ inch thick stock. It is used to drill out holes for 3/8 inch dowels and can be used to drill out multiple rows or patterns for thicker stock.
In the image below you will see that the guide allows you to drill five 3/8 inch dowel holes that are 3/4 inch on center, 5/8 inch from each edge, and has a centering slot aligned with the center of the middle guide.
The Jessem 1/4 Inch Doweling Guide:
The 1/4 inch doweling guide is typically used for 1/2 inch to 3/4 inch thick stock where a smaller 1/4 inch dowel is required. It has seven drill guides spaces at 1/2 inch on center. The guides start 5/8 inch from each edge. It has a centering slot aligned to the middle guide. It can be set to drill out multiple rows and patterns for thicker stock.
The Jessem 1/2 Inch Doweling Guide:
The 1/2 inch dowel guide is used for stock that is 1-1/2 inch thick or more. It has three drill guides spaces at 1 inch on center. The guides start 1-1/8 inch from each edge. It also has a centering slot aligned to the middle guide. It can be set to drill out multiple rows and patterns for thicker stock.
