Quoted from KevinCPR:

Just interjecting for sake of terminology. I see people calling this wire on the bottom of the playfield "the ground braid" in seemingly thousands of places. It's becoming common terminology, and it's scarier with all the newcomers to the hobby trying to learn the circuits...
The stapled bare wire going to all the bulbs on the bottom of the playfield IS NOT A GROUND BRAID.
It is the + (positive) voltage supply.
The bulb driver board is your ground. The transistors **pull the bulbs to ground** to light them.
Just saying, if this becomes commonspeak, it will be shocking the number of people who tie something, or connect something, to that line - a tester, a mod, a jumper wire, a bulletproofing wire, etc... thinking it is ground... at best you're going to pop a fuse. At worst, something is going to fry.
REMEMBER: The ground braid is in the *cabinet* (an actual braid - not a wire - that goes to metal things to keep them grounded - like the coin door, the cabinet rails, the plunger, etc).
Know the difference!