Since the beginning of 玄空 legacy in ancient times, the Fengshui principles and methods are based only on 《法诀》 and 《眼诀》. Thereafter, in order to prevent the lost of knowledge over time, and to allow later generations to validate their Fengshui principles and methods against, Classics are written and passed down.

These Classics are the Qing Nang Jing 《青囊经》, Qing Nang Xu 《青囊序》, Qing Nang Ao Yu 《青囊奥语》, Tian Yu Jing 《天玉经》 and Du Tian Bao Zhao Jing 《都天宝照经》, which are later collectively named as the 《书诀》. Together, 《书诀》, 《法诀》and 《眼诀》 are known as 《玄空三诀》, or literally, the Three Principles and Methods of Xuan Kong, where:

《书诀》 is meant for one to validate his or her Fengshui principles and method (演经正诀) against each and every line of the Classics, to verify that his or her Fengshui principles and methods are indeed the correct ones.

《法诀》 is Xuan Kong Liu Fa (玄空六法), comprising of《一心, 四决 and 一法》.  一心 is Xuan Kong (玄空), 四决 is Ci Xiong Jue (雌雄诀), Jin Long Jue (金龙诀), Ai Xing Jue (挨星诀), Cheng Men Jue (城门诀), and 一法 is Tai Sui Fa (太岁法).

《眼诀》 is the principles and methods of Landforms (形峦). It is also the prerequisite for understanding Ci Xiong Jue, where Ci Xiong Jue is the recognition of the formation (形局) form by the physical Landforms of Yin (雌) and Yang (雄) nature. Hence, 《眼诀》 is the most important yet fundamental aspect of Fengshui principles and methods!

As mentioned before, the emphasis for Fengshui must be on Landforms (形栾) first. If one fails to recognize the Landforms correctly, all other efforts thereafter are futile. In fact, in the application of Xuan Kong Liu Fa (玄空六法), Ci Xiong Jue (雌雄诀) is the first usage sequence, as reflected in the first sentence of the Fengshui Classic Qing Nang Xu (青囊序).

Yet, there are still many practitioners who use Li Qi (理气) as the basis first, followed by Landforms. This is wrong. If such is the case, the opening of the Fengshui Classic 《青囊序》 will not be “杨公养老看雌雄…”! One must remember, 因地定局，因局下卦，因卦起星!

Having acquired 《眼诀》, one will know, for example, how different types of formations are formed by the physical Landforms of Yin and Yang nature, whether for landed or high-rise properties; what real Qi Mouth (气口) and Water Mouth (水口) really are, their differences, and how to recognize them; understand why T-junction or intersection (交叉口) is not necessary the Qi/ Water Mouth and why in many cases, they may not even play a part in the Fengshui of the property; understand what and where is Cheng (城) and Men (门), and hence the Cheng Men (城门); etc.