No, this isn’t about weird relationships between people and their tools. Though woodworkers might be a fine group to have that discussion. The idea of tool slaving is to acquire a set of tools in sizes which will complement each other. I ran into this the other night when I needed to make some small square holes. The plan called for 3/8″ holes, but I’d have settled for 5/16″. Turns out even though I have two chisels with the label “3/8″ on them, I don’t have any chisels that are actually 3/8” wide.
I have a nice set of Irwin bits for my brace which are all in true imperial (based on inches and feet) sizes. The problem is that all my chisels are in metric sizes. In addition to the metric size labels on each chisel, the maker also put the inch-equivalent size. But those are misleading. The 6mm chisel is very close to 1/4″ wide. It’s actually a hair shy. But then the chisel that’s supposed to be close to 3/8″ is actually 10mm wide. It just takes a little bit of reckoning to see that if 6mm is 1/4″ then 10mm is not 3/8″. And you can see this in the second photo, how the bottom chisel is wider than 3/8″.
I know of one other set of tools where slaving is important. That being for rail and stile doors. The tool that cuts the groove for the panel (a plow plane or router bit) should be the same size as the tool that cuts the mortises at the end of each stile (a chisel, router bit, or mortiser chisel).
I know of only a couple high-end chisels available in imperial sizes. And many vendors list the chisels they sell in imperial sizes, but they’re actually metric. Normally I would be a little more open-minded about the metric system. But, except for my unfortunate chisel collection, all my other tools are sized in inches.