PIP: The use of gonane nomenclature can be traced back to the early 1960s, when the first compound submitted for trial, the carbon-18 homologue of the anabolic agent Nilevar was named bisnortestosterone. However, the confusion about how this compound was named led scientists to introduce the gonane system for naming bisnortestosterones and structurally related compounds. Use of the terms gonane and estrane to classify the levonorgestrel and norethindrone families of progestins, respectively, originates from the systemic name of these compounds. Levonorgestrel is 17beta-hydroxy-17alpha-ethinyl-13beta-ethyl-4-gonen-3-one, while norethindrone is 17beta-hydroxy-17alpha-ethinyl-4-estren-3-one. The term gonane signifies that levonorgestrel and the related progestins are a separate class of steroids that differ from other steroids. Levonorgestrel and the related progestins form a structural category of 18-homologated 19-nortestosterones. Thus, it would be better to categorize levonorgestrel and the structurally related progestins such as desogestrel, norgestimate, gestodene as carbon-18-homologated 19-nortestosterones. Alternatively, it is simpler to refer to these compounds as the levonorgestrel family of progestins. In a similar manner, norethindrone and the related progestins can be referred to as the norethindrone (norethisterone) family of progestins.