Not to be _that_ guy, but it was technically -48V DC.

Honestly, that was pretty surprising to me when I had to work with some telco equipment a couple of decades ago. To this day, I don't think I've encountered anything else that requires negative voltage relative to ground.

Yes, and that tiny little difference can cost you a lot of expensive gear if you run it off the battery and plug in a serial port or something like that. You'll also learn first hand what arc welding looks like without welding glass.

Some old guitar effects used -9V DC.[1] And the convention with guitar effects power adapter is the barrel is center negative (which is motivated with facilitating easy wiring of the socket's switch to connect to a 9V battery inside).

[1] https://www.analogisnotdead.com/article26/what-is-going-on-w...

Can you explain why it's -48 VDC as opposed to 48 VDC with the + and - inputs mislabeled?

Lots of amplifier circuits need a bipolar supply: both positive and negative voltages with respect to ground.

RTL and DTL both needed negative-voltage relative to ground, as do many analog circuits.

Is that something other than a labelling convention? Is ground actually connected to a earth stake?

positive ground used to be in all cars. When they went from 6 volts to 12 the disadvantages became appearant fast and so everyone went negative ground then (mid 1950s). I am not clear why positive ground was bad (maybe corrosion?)

Check out older English cars.