esd - What exactly is grounded through motherboard standoffs?

My micro-ATX motherboard is mounted to brass standoffs inside an unpainted metal case.

What part of the motherboard is electrically bonded to the case through these standoffs?

1. Only the metal traces around the screw holes?


2. Or the whole ground-plane of the motherboard?

The ground-plane is the common ground for all circuit grounds on the motherboard, right? It's a copper layer inside the multilayer PCB, isn't it?

If the standoffs are bonded to the ground-plane, then i have another question.

1. Is the case grounded to the motherboard?


2. Or is the motherboard grounded to the case?