The circles are 5.5mm and the stands are 12mm, both made of Lauan plywood. The groove width is 6mm, so there is a slight gap. The circles are cut to improve stability. Please create wonderful works with your favorite designs. The finish is done with a fret saw and a grinder, so please allow for some size variations. There may be cracks on the back side of the cut or brown spots. Please refrain from purchasing if you are concerned about minor details or if you are seeking perfection. Please refrain from price negotiations.