The I Ching doesn’t explicitly specify who is who. In my opinion, their relationship is derived from:
1) According to Xi CI (繫詞), Volume 1, Chapter 9, the sky (is designated as) 1, the earth (is designated as) 2, the sky 3, the earth 4, the sky 5, the earth 6, the sky 7, the earth 8, the sky 9, and the earth 10, signify that masculinity is odd numbers while femininity is even numbers.
2) According to Shuo Gua (說卦), Chapter 1: 參天兩地而倚數，觀變於陰陽，而立卦,
A) it can be interpreted as: to triple the sky and double the earth so as to count on the numbers, to observe changes between the feminine and the masculine, and then to create a hexagram.
B) it can be understood as:3 x the sky: the sky 1 plus the sky 3 and the sky 5 equals 9.2 x the earth: the earth 2 plus the earth 4 equals 6.9 - the sky 1 = 8 6 + the sky 1 = 7 6, 7, 8, and 9 are four numbers created from divination with yarrows.
3) The 7th line of Hex 1 (Qian) and 2 (Kun) are called ‘to use 9’ and ‘to use 6’, respectively, while divination comes out with all six 9s or six 6s, wherein the masculine dragons are becoming submissive and do not fight for leadership any longer, and wherein femininity is changing to masculinity since the masculine is the desired end of the feminine.
Therefore 9 is taken for the old masculine while 6 is the old feminine, and 7 is taken for the young masculine while 8 is taken for the young feminine.