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The Gap Concept is a widely understood principle amongst professional poker tournament players that figures this way, "you need a better hand to call a raise with than you would need to open the betting yourself". For a simple example, if I am in middle position with AJs, and it has been folded to me, this is a good raising opportunity. However, holding the same hand on the button, facing a good sized raise from early position, this would generally be a fold, save for other conditions.