As a divinatory line, nine at the first place of Decrease reads within the hexagram's overall concern with sincerity in one who practices decrease brings great good fortune, freedom from error, firmness and correctness maintained, advantage in every movement, and even in the use of two small dishes for sacrifice. The line occupies a place proper to its own nature in the classical doctrine that odd places favour a firm, yang line and even places favour a yielding, yin line. Its own text, that leaving off one's own business and hastening to help another, judging for himself how far to go without doing himself injury, is the particular fortune assigned to this one stage of the situation, not to the hexagram as a whole.
The first line corresponds by classical convention with the fourth line, and the two correspond here, since one line is yin and the other yang. Read together with the trigram it belongs to, Dui in the lower position of the figure, the line marks the opening of the situation, before the trigram above it has any part in the reading, one stage in the six stage movement the hexagram traces from its first line to its last.
A change at this line, from yang to its opposite, is the mechanism the tradition uses to move from one hexagram to another. Flipping the first line of Decrease turns Dui into Kan while the other trigram, Gen, stays as it is, which yields Decrease's changing partner, Youthful Folly (King Wen 4), the hexagram this line is recorded as transforming into.