Despite doing quite a lot of legal writing, I almost never use the §± key, which is under Escape on a macOS keyboard.

I tend to type by muscle memory, and due a recent finger injury (...mandolines, or fancy chopping machines, are not for me) hitting right-shift and hyphen to get an underscore (_) is actually quite painful.

I've just remapped this using Karabiner-Elements, so when I hit the §± button, I now get _. The shift modifier still works, so hitting Shift + § still outputs ±.

Here's how to do it:

1. Install Karabiner-Elements from https://karabiner-elements.pqrs.org/ and go through the various macOS security prompts. (Karabiner-Elements is open source, with the code on Github.)
2. Choose "Add your own rule" and paste in the following JSON: