The "bolt pattern" determines the number and position of the bolt holes on the wheel to line up with your vehicle's studs on the mounting hubs. The bolt holes are spaced evenly about the bolt hole circle. "Wheel studs" are the bolts that are on your mounting hub and are used along with lug nuts to attach the wheel to the car. The "bolt hole circle" is the circle that the center of each bolt aligns with. The second number in a bolt pattern is the diameter of this circle. The bolt circle has the same center point as the mounting hub to ensure that the wheel will be concentric with the mounting hub. The bolt circle's measurement is called the bolt circle diameter (BCD), also called the "pitch circle diameter" (PCD).