The Big Box stores around here don't have taps.
You need a tap to cut the threads in the 8.5mm hole that the rear derailleur threads into. It is a 10mm tap ... however, not just any 10mm tap will do. The upper tap in the foto is 10mm x 1.5mm (I didn't know what the thread count on it was since it isn't marked, so I tried it on a piece of scrap metal ... too course for the derailleur threads). At this point I searched the internet and on Sheldon Brown's site it specified that the tap had to be 10mm x 1.0mm. I went to the nearest BIG city (I live in a tiny town with cows) ... where I found an industrial hardware store ... that just happened to have the bottom tap in stock (this saved me about $6 difference from the cost of buying it and having it shipped from Amazon.com).
The new tap worked absolutely fine. I tapped the hole in the dropout and attached the derailleur.