Most wheel spacers bolt onto a truck's axles or hubs, and include studs that wheels are mounted on. Adapters, on the other hand, also have studs, but they're arranged in a different bolt pattern.

Here's What Happens When You Put 12 Feet Of Wheel Spacer On Your Pickup Truck. By Andrew P. Collins April 6, 2019 10:30 am EST. WhistlinDiesel/YouTube.