It should be the length of the turn. I'm not sure if it is right.
20 stands for that the length of the brick is 20CM, there is only a kind of 620 brick for masonry. Simply say: 1. 620 and 320 these two specifications of bricks are needed simultaneously if it is 4 meters kiln that should be built. When the diameter of the kiln is 6M. Other specifications of kiln can be built by useing these two kinds of bricks in different proportions. 3M kiln can be built only by using 320 brick, its proportion of use is 16. 3 stands for kiln's diameter
