In the early history of the study of radioactivity, the different natural isotopes of radium were given different names. In this scheme, 223Ra was named actinium X (AcX), 224Ra thorium X (ThX), 226Ra radium (Ra), and 228Ra mesothorium 1 (MsTh1). When it was realized that all of these are isotopes of the same element, many of these names fell out of use, and "radium" came to refer to all isotopes, not just 226Ra. Some of radium-226's decay products received historical names including "radium", ranging from radium A to radium G, with the letter indicating approximately how far they were down the chain from their parent 226Ra. Radium emanation = 222Rn, RaA = 218Po, RaB = 214Pb, RaC = 214Bi, RaC1 = 214Po, RaC2 = 210Tl, RaD = 210Pb, RaE = 210Bi, RaF = 210Po and RaG = 206Pb.