Thunder and lightning both arrive, forming the image of Abundance; brightness and movement support each other, attaining the Dao of Abundance. Without brightness there is nothing to illuminate; without movement there is nothing to execute.
They need each other like form and shadow, relying on each other like exterior and interior. Nine at the Beginning is the beginning of brightness; Nine in the Fourth Place is the beginning of movement. They ought to depend on each other to fulfill their function.
Thus, although they are equal, they correspond: their positions correspond, and their applications support each other. Therefore, Line One calls Line Four its 'matching master'—the one with whom it matches.
Although 'match' implies equal status, it means approaching the other, as in 'matching Heaven' or 'matching the noble person.' Hence Line One refers to Line Four as a 'match,' while Line Four refers to Line One as an 'equal.' 'Though equal, no blame'—'xun' means equal.
Those who correspond under Heaven are usually not equal rivals, as when Yin responds to Yang, the yielding follows the firm, or the inferior attaches to the superior. If they were rivals, how could they follow each other?
Only in the initial and fourth lines of Abundance do their functions support each other and their responses complete each other; thus, although both are equally Yang and firm, following each other incurs no fault.
For without brightness, movement has nowhere to go; without movement, brightness has nowhere to be applied.
Supporting each other to achieve usefulness, like being in the same boat makes the Hu and Yue peoples of one heart, or sharing hardship makes enemies cooperate: the momentum of circumstances makes it so.
Going and following each other enables them to achieve abundance; hence it says 'there is appreciation'—there is something worthy of praise. In other hexagrams, they would not yield to each other and estrangement would arise.