Developed in South Africa in early 1950s. Has not been used much in the past, but is now starting to enjoy some popularity, particularly where salinity can be a problem. Trees on X639 are fairly similar in most characteristics such as internal quality, tree shape, and yield to Troyer citrange, but growth is slightly slower. Seedling growth in the nursery is also slightly less vigorous than the citranges. X639 has been reported to be comparable to Swingle citrumelo and slightly susceptible to Phytophthora citrophthora (tests conducted by Dr. Pat Barkley in Australia).