In “The New Sunspot Numbers” (Oct. 2016 QST, pp. 38-41), Carl Luetzelschwab, K9LA, reviews the history of sunspot numbers and how they are calculated, as well as explains a new formulation of sunspot numbers in use by the Royal Observatory of Belgium. Sunspot numbers are important because they are correlated with the atmospheric ionization on Earth. The sunspot number is used in propagation calculations and to study long-term trends in solar activity. When using sunspot numbers for such calculations, it is important to understand which version index is the correct one for your application.