All pure breeds were originally hybrids derived from the original wild Jungle Fowl of which there are two types I believe. Different strains of bird were bred and selected for desired traits and by continous reselection the lines are "straitened" until all offspring come true to type. Therefore after a number of generations of breeding pure within a cross a new pure breed can be established.
Just consider the mediteranean light breeds, Leghorns, Minorcas, Anconas etc they were all established in geographical regions by being bred continously in that region without importing fresh genes from other regions.
I think 7 generations of being bred pure without an outcross establises the breed, but it still has to be recognised as a breed.
At least that's how I understand it.