Here's what I was taught:

Both of these bits are 1-1/2" auger bits. The one on the top/left is called a "Scotch pattern". The one on the bottom/right is a "Jennings pattern".




Both are double spurred bits. Both have flat cutters that are approximately parallel to the work surface that lift off the chips. Both have lead screws to pull the bit into the wood. The difference is in the location of the spurs. The Jennings spurs hang down below the cutters. They score the circular edge of the chip before the cutter lifts it out. That makes for a cleaner hole. The Scotch spurs stick up above the cutters, and cut the circular edge of the chip at the same time the cutters are lifting up the chip. Becuase it does not have to drive the spurs into the uncut wood, it requires less effort to turn, but it does not cut as clean a hole. In general (I repeat, in general), the Irwin pattern works best with power tools, and the Scotch with hand tools, as the added power for turning the Jennings comes from the motor, not the arms. It's probably not a big deal for a boring machine, but on a T-Auger, every little bit helps!

Note also that in my examples, the Scotch pattern has a more aggressive screw thread, requring fewer revolutions to bore the same depth. Again, with a power tool, more revolutions doesn't make much difference. I think the aggressive screw pattern is also more appropriate for softwoods, the finer pitch thread for hardwoods.

Finally, I agree with Dan, that you should track down Jim Rogers. He knows his stuff, and sells high quality tools. CB.
CB.