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Way, way back in early July, Brad Treliving referred to the NHL as a "league of pairs". He wasn't talking about defencemen there; rather, he was referencing the composition of forward lines.
You find two players who compliment one another perfectly, and that's your base for that particular line. Repeat four times, until you've got four pairs of players who can really feed off one another's game, and then add the final component to their line.
The best exhibit of this the past season on the Flames was the combination of Johnny Gaudreau and Jiri Hudler.