Hello, I wanted to make a wallet and have been reading about leatherwork. Based on what I read (correct me if I'm wrong), a 3mm diamond stitching chisel would be a good option for a wallet. However, when I decided to look up these tools I was confused as the mm number does not always equal the stitch per inch number.

Tandy Leather Craftool Pro Line Diamond Stitching Chisel 3mm = 5 SPI Weaver Leather Diamond Stitching Chisel 3mm = 9 SPI Seiwa Diamon Stitching Chisel 3mm = 9 Holes per Inch. Is this the same as SPI? Craft Sha Pricking Iron 3mm = 5 Holes per Inch. Although the description does say that a 1.5mm would make 9 Holes per Inch.

Also, GoodsJapan has the description for the Seiwa and Craft Sha tools as both a diamond chisel and a pricking iron. I thought a diamond chisel and a pricking iron were different. Am I wrong? Is the diamond just the shape of the hole?

So, if any kind soul can explain why the numbers might differ and what they would recommend to use for a wallet. Thank you!