ERU Classification System (ECS): (developed by the Endlers R Us website and knowledge base) There are currently three recognized types of endlers. Class N Endler's livebearers can be shown to have originated from their native waters in Venezuela. This includes all progeny from said endlers. Class P endlers are endlers that contain all known characteristics of the breed, but whose origins cannot be determined. Class K endlers are those who have been crossbred to other livebearing species, such as Poecilia reticulata. Each class has its own strains. Some examples of strains are the black bar strain found in Class N livebearers, the Dutch green found in Class P and the hybrid tiger found in Class K. As can be observed, the strains in Class N are similar to those in Class P, though those in Class P tend to be bred for a specific attribute such as a green body rather than overall color. Class K endlers have morphological differences, with longer fins that tend to appear similar to a guppy's fins. The description of the strains are taken directly from the ERU Strain Registry's List of Registered Strains. (taken from the site ultimateendlers)