If the Five Elements are the basic elements that constitute the universe, then the Heavenly Stems and Earthly Branches are the symbolic system that records the movement and transformation of these elements. Commonly abbreviated as "Gan-Zhi" (Stems and Branches), this system represents an original Chinese method of timekeeping and serves as the core tool for Bazi chart construction. Without the Stems and Branches system, Bazi destiny analysis would not exist.

There are ten Heavenly Stems: Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, and Gui. There are twelve Earthly Branches: Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, and Hai. The Stems and Branches are paired in a fixed sequence, beginning with Jia-Zi and ending with Gui-Hai, forming sixty combinations known as the Sexagenary Cycle (60 Jiazi). These sixty combinations repeat in an endless cycle, used to mark years, months, days, and hours, constituting a precise temporal coordinate system in ancient China.

The Heavenly Stems have strict correspondences with the Five Elements and Yin-Yang. Jia, Bing, Wu, Geng, and Ren are Yang Stems; Yi, Ding, Ji, Xin, and Gui are Yin Stems. Jia and Yi belong to Wood; Bing and Ding belong to Fire; Wu and Ji belong to Earth; Geng and Xin belong to Metal; Ren and Gui belong to Water. This correspondence means that each Stem simultaneously possesses dual attributes of Yin-Yang and the Five Elements. For example, Jia Wood is Yang Wood, symbolizing a towering tree with the qualities of upward growth and upright resilience; Yi Wood is Yin Wood, symbolizing flowers and vines with the qualities of flexibility and climbing attachment.

The correspondence between Earthly Branches and the Five Elements is more complex, because each Branch contains not only its primary element (Main Qi) but also one or more hidden Stems, known as "Hidden Stems" (Cang Gan). For example, Yin contains Jia Wood, Bing Fire, and Wu Earth; Si contains Bing Fire, Wu Earth, and Geng Metal. This mechanism of Hidden Stems makes the Five Element attributes of the Branches richer and more multidimensional, providing the possibility for refined analysis in Bazi destiny analysis.

Among the Branches, there also exist complex relationships such as the Six Combinations, Six Clashes, Three Combinations, and Three Punishments. The Six Combinations are: Zi-Chou combines into Earth, Yin-Hai combines into Wood, Mao-Xu combines into Fire, Chen-You combines into Metal, Si-Shen combines into Water, and Wu-Wei combines into Earth, representing harmonious unions between Branches. The Six Clashes are: Zi-Wu clash, Chou-Wei clash, Yin-Shen clash, Mao-You clash, Chen-Xu clash, and Si-Hai clash, representing direct conflicts between Branches. The Three Combinations are: Shen-Zi-Chen combines into Water, Hai-Mao-Wei combines into Wood, Yin-Wu-Xu combines into Fire, and Si-You-Chou combines into Metal, representing three Branches converging into a powerful Five Element force. The Three Punishments refer to punitive relationships among Branches, such as the Yin-Si-Shen punishment and the Chou-Xu-Wei punishment, often representing contradiction and harm.

In Bazi destiny analysis, a person's birth year, month, day, and hour are each represented by one Heavenly Stem and one Earthly Branch, totaling eight characters—hence the name "Eight Characters" (Bazi). For example, a person born in the year Jia-Zi, month Bing-Yin, day Wu-Chen, and hour Geng-Wu would have the Bazi chart: "Jia-Zi Bing-Yin Wu-Chen Geng-Wu." These eight characters contain the cosmic energy information at the moment of that person's birth. The destiny analyst infers the trajectory of that person's life by analyzing the relationships of generation and restriction, clash and combination, and punishment and harm among these eight characters.

The Stems and Branches system is not merely a tool for recording time; it is a profound grasp of the rhythms of the universe by ancient Chinese thinkers. The Heavenly Stems govern the external and the active, representing manifest and superficial information; the Earthly Branches govern the internal and the passive, representing hidden and essential information. The Stems are like the branches of a tree, while the Branches are like its roots—the two complement each other, together forming the symbolic foundation of Bazi destiny analysis. Mastering the basic knowledge of the Heavenly Stems and Earthly Branches is the prerequisite for reading a Bazi chart.

如果说五行是构成宇宙的基本元素，那么天干地支就是记录这些元素运动变化的符号系统。天干地支，简称"干支"，是中国古代独创的纪时方法，也是八字命理学排盘的核心工具。没有干支系统，就没有八字命理的存在。

天干共有十个，依次为：甲、乙、丙、丁、戊、己、庚、辛、壬、癸。地支共有十二个，依次为：子、丑、寅、卯、辰、巳、午、未、申、酉、戌、亥。天干与地支按固定顺序两两相配，从甲子开始，到癸亥结束，共组成六十个组合，称为"六十甲子"。这六十个组合循环往复，用以纪年、纪月、纪日、纪时，构成了中国古代精密的时间坐标体系。

天干与五行、阴阳有着严格的对应关系。甲、丙、戊、庚、壬为阳干，乙、丁、己、辛、癸为阴干。甲、乙属木，丙、丁属火，戊、己属土，庚、辛属金，壬、癸属水。这种对应使得每一个天干都同时具有阴阳和五行双重属性。例如，甲木为阳木，象征参天大树，具有向上生长、刚直挺拔的特性；乙木为阴木，象征花草藤蔓，具有柔韧弯曲、依附攀援的特性。

地支与五行的对应则更为复杂，因为每个地支中除了本气之外，还藏有一个或几个天干，称为"地支藏干"。例如，寅中藏甲木、丙火、戊土；巳中藏丙火、戊土、庚金。这种藏干机制使得地支的五行属性更加丰富和立体，也为八字命理的精细推断提供了可能。

地支之间还存在六合、六冲、三合、三刑等复杂关系。六合是指子丑合土、寅亥合木、卯戌合火、辰酉合金、巳申合水、午未合土，代表地支之间的和谐结合。六冲是指子午冲、丑未冲、寅申冲、卯酉冲、辰戌冲、巳亥冲，代表地支之间的正面冲突。三合是指申子辰合水局、亥卯未合木局、寅午戌合火局、巳酉丑合金局，代表三个地支汇聚成一种强大的五行力量。三刑则是指地支之间的刑伤关系，如寅巳申三刑、丑戌未三刑等，往往代表矛盾与伤害。

在八字命理中，一个人的出生年、月、日、时分别用一个天干和一个地支来表示，共八个字，故称"八字"。例如，某人出生于甲子年、丙寅月、戊辰日、庚午时，其八字就是"甲子 丙寅 戊辰 庚午"。这八个字蕴含了此人出生时刻的宇宙能量信息，命理师通过分析这八个字之间的生克制化、刑冲合害关系，来推断其一生的命运轨迹。

干支系统不仅是时间记录工具，更是古人对宇宙节律的深刻把握。天干主外、主动，代表显露的、表象的信息；地支主内、主静，代表隐藏的、本质的信息。天干如树之枝干，地支如树之根系，二者相辅相成，共同构成了八字命理学的符号基础。掌握天干地支的基本知识，是读懂八字命盘的前提条件。