The practice of making the cheapest bid, when responding or rebidding with two or three 4-card suits. This principle features in many bidding styles, with exceptions in some cases about the quality of biddable suits or the need to show a major suit.
If partner opens 1♦, you should respond 1♥, the cheapest bid. This facilitates finding a 4-4 fit, if one should exist, in any one of your four- card suits.