Some students argue among themselves whether Tai Sui of the year covers 30 degrees or 15 degrees of the luopan.
Qing Nang Xu 青囊序 says:
In the luopan, the Xian Tian divisions are the twelve branches.
In Hou Tian, the stems and the corner trigrams are also used.
This means that the Heavenly bodies occupy and define the twelve palaces along the yellow path. When it is mapped onto the earth, in addition to the twelve branches, the eight stems (Wu and Ji go to the center to become the Huang Ji 皇極) are added to assist the branches. Moreover, the four trigrams Qian, Gen, Xun and Kun are also added to make it up to 24 paths occupying 15 degrees each.
Xian Tian is the body while Hou Tian is the application. Fundamentally, Tai Sui covers 30 degrees but in Feng Shui practices, we use the branches of the 24 mountains to define the location of the Tai Sui. 7.5 degrees from each side of the branch will be occupied by assisting forces of the Tai Sui.