The bolt pattern or bolt circle is the diameter of an imaginary circle formed by the centers of the wheel lugs. Bolt patterns can be 4, 5, 6, or 8 lug holes. A bolt circle of 4x100 would indicate a 4 lug pattern on a circle with a diameter of 100mm.
The diagram indicates the proper measuring methods. 6 lug wheels are measured like the 4 lug wheels.