line breeding is a way to help set, desirable traits, with line breeding you breed animals that are related, but you also introduce genes from "other lines", into the genetic mix, it takes a long time to fix desirable traits this way, but doing so lowers the risk of problems, assosiated with repeated in breeding, with tight line breeding, you might find the same 3 or 4 dogs, showing up numerous times in a 5 generation pedigree