The RHS Classification is “2 or 3 (b or c)” So it appears the division varies.
White Nun, 2 W-, Walter T. Ware, England, 1914
Photo #31732 Robert Johnson, USAWhite Nun, 2 W-, Walter T. Ware, England, 1914
Photo #31731 Robert Johnson, USAWhite Nun, 2 W-, Walter T. Ware, England, 1914
Photo #31730 Robert Johnson, USA