Found this old post in our old forum by bro minghan. Thought it would be good sharing for all, especially for those who breed and develop lines.
"In genetic parlance the initial mating in a sequence is called the parental (P 1 for short) generation. Subsequent generations are called filial generations, the first filial is called the F 1, the next the F 2 and so on." - Gene A. Lucas
in short:-
P1 = Parental Generation 1
F1 = Progeny (fry) of P1
F2 = Progeny of F1xF1
F3 = Progeny of F2xF2
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F1xF1=P2
F2xF2=P3