Nowadays the world popular Sam Yuan Xuan Kong Flying Star feng shui is, in fact originated from.the <shen xuankong science> refined and written in China late Ching dynasty by master Shen Zhu Ren. 
 
In this xuan kong classical literature, there is a chapter talking about the art of "sum-of-ten" method using flying stars. The sum-of-ten refers to the the scenario where:
 
  • the sector's yun star and water star add up to 10 symbolising excellent prosperous luck. For example, three yun gui-ding facing. Or 
  • the sector's yun star and mountain star add up to 10 symbolising good health and life safety. For example,.seven yun zi-wu facing 
 
As per the explanatory notes written by the student follower of master Shen: 
 
From two to eight yun, the sam yuan (heaven, earth and human yun) all have the "on mountain on water" good flyingstar setting. Though the one yun and nine yun both flyingstar settings do not have "on mountain on water" settings, there are still "sum-of-ten" settings to be used. For example, one and nine yun for qian-xun or si-hai (mountains for facing / sitting)
The "sum-of-ten" settings are also available in the:
 
  • two and eight yun for chou-wei
  • three and seven yun for zi-wu, gui-ding
  • four and six yun for.geng-jia
 
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《沈氏玄空學》 合十局, 是指  向與運星合十 （如三運癸山丁向 ）, 又或是 山與運星合十 （ 如七運子山午向）
 
這可翻看其《論夫婦合十》篇, 內云：江志伊謹案： ………………  二運至八運。天地人三元均有旺山旺向, 而一九兩運獨無, 實為缺憾。今考夫婦合十, 則一九運有乾巽已亥, 二八運有丑未, 三七運有子午癸丁, 四六運有庚甲, 三元九運中全局十者, 共得二十四山向, 是可補旺山旺向之缺憾矣, 願學者擇而用之可也。