Yue Ling
The term YueLing 月令 is made of 2 words, Yue 月 which means month and Ling 令, which means command. Historically, YueLing refers to the different proceedings of the government for different months through the year. Such was recorded in 礼记，The book of rites written around 200 BC. 令 'command' refers to the command from the government to the pheasants to carry out certain activities such as sowing, harvesting, repairing castle wall etc etc.
In Bazi, YueLing refers to the month commander, the entity that takes charge during a certain period of the month. There are 12 months in Bazi, beginning with Yin, which also marks the beginning of spring to Chou, which marks the end of the year. From Yin to Chou, the Yin and Yang of the 5 elements take turns in ruling the month.
SMTH writes:
轻清者为十干，主禄，谓之天元，
The ones that are light and pure are the 10 stems, they represent 'fortune', named as Heavenly Origin
重浊者为十二支，主身，谓之地元，
重浊者为十二支，主身，谓之地元，
The ones that are heavy and mixed are the 12 branches, they represent 'body', named as Earthly Origin
天地多正其位，成才于两间者，乃所谓人也。
天地多正其位，成才于两间者，乃所谓人也。
The ones that reside and work between heaven and earth are human.
故支吕所藏者主命，谓之人元，名为司事之神，以命术言之为月令
故支吕所藏者主命，谓之人元，名为司事之神，以命术言之为月令
Thus, the ones stored inside the branches are called Human Origin, its name is the God that takes charge, from the art of life perspective, the name is Month Command.
Solar Terms
To understand the concept of YueLing, knowledge about the 24 Solar Terms is necessary. Solar Terms were used by the ancient Chinese to mark certain astronomical or natural event that happens through the year. A year is divided into 4 seasons 季，each season is divided into 6 terms 节 and each term is further divided into 3 pentads 候。Briefly, there are 4 seasons, 24 terms and 72 pentads in a year.
The table below shows the name of the 24 solar terms and the date they happen. (from wikipedia)
| Longi- tude | Usual translation | Chinese name ¹ | Japanese name | Korean name ² | Vietnamese name | Date ³ | Remark |
|---|---|---|---|---|---|---|---|
| 315° | start of spring | 立春 lìchūn | 立春 risshun | 입춘 (立春) ipchun | Lập xuân | Feb 4 | |
| 330° | rain water | 雨水 yǔshuǐ | 雨水 usui | 우수 (雨水) usu | Vũ thủy | Feb 19 | more rain than snow |
| 345° | awakening of insects | 驚蟄 (惊蛰) jīngzhé | 啓蟄 keichitsu | 경칩 (驚蟄) gyeongchip | Kinh trập | Mar 6 | lit. awakening of hibernating insects. See the note at Chinese calendar#Solar term. |
| 0° | vernal equinox | 春分 chūnfēn | 春分 shunbun | 춘분 (春分) chunbun | Xuân phân | Mar 21 | lit. spring division (or center) |
| 15° | clear and bright | 清明 qīngmíng | 清明 seimei | 청명 (清明) cheongmyeong | Thanh minh | Apr 5 | time for tending graves |
| 30° | grain rain | 穀雨 (谷雨) gǔyǔ | 穀雨 kokuu | 곡우 (穀雨) gogu | Cốc vũ | Apr 20 | lit. grain rain: rain helps grain grow |
| 45° | start of summer | 立夏 lìxià | 立夏 rikka | 입하 (立夏) ipha | Lập hạ | May 6 | |
| 60° | grain full | 小滿 (小满) xiǎomǎn | 小満 shōman | 소만 (小滿) soman | Tiểu mãn | May 21 | grains are plump |
| 75° | grain in ear | 芒種 (芒种) mángzhòng | 芒種 bōshu | 망종 (芒種) mangjong | Mang chủng | Jun 6 | lit. awns (beard of grain) grow |
| 90° | summer solstice | 夏至 xiàzhì | 夏至 geshi | 하지 (夏至) haji | Hạ chí | Jun 21 | lit. summer extreme (of sun's height) |
| 105° | minor heat | 小暑 xiǎoshǔ | 小暑 shōsho | 소서 (小暑) soseo | Tiểu thử | Jul 7 | |
| 120° | major heat | 大暑 dàshǔ | 大暑 taisho | 대서 (大暑) daeseo | Đại thử | Jul 23 | |
| 135° | start of autumn | 立秋 lìqiū | 立秋 risshū | 입추 (立秋) ipchu | Lập thu | Aug 8 | |
| 150° | limit of heat | 處暑 (处暑) chǔshǔ | 処暑 shosho | 처서 (處暑) cheoseo | Xử thử | Aug 23 | lit. dwell in heat |
| 165° | white dew | 白露 báilù | 白露 hakuro | 백로 (白露) baekro | Bạch lộ | Sep 8 | condensed moisture makes dew white |
| 180° | autumnal equinox | 秋分 qiūfēn | 秋分 shūbun | 추분 (秋分) chubun | Thu phân | Sep 23 | lit. autumn division (or center) |
| 195° | cold dew | 寒露 hánlù | 寒露 kanro | 한로 (寒露) hanlo | Hàn lộ | Oct 8 | |
| 210° | frost descent | 霜降 shuāngjiàng | 霜降 sōkō | 상강 (霜降) sanggang | Sương giáng | Oct 23 | appearance of frost and descent of temperature |
| 225° | start of winter | 立冬 lìdōng | 立冬 rittō | 입동 (立冬) ipdong | Lập đông | Nov 7 | |
| 240° | minor snow | 小雪 xiǎoxuě | 小雪 shōsetsu | 소설 (小雪) soseol | Tiểu tuyết | Nov 22 | |
| 255° | major snow | 大雪 dàxuě | 大雪 taisetsu | 대설 (大雪) daeseol | Đại tuyết | Dec 7 | |
| 270° | winter solstice | 冬至 dōngzhì | 冬至 tōji | 동지 (冬至) dongji | Đông chí | Dec 22 | lit. winter extreme (of sun's height) |
| 285° | minor cold | 小寒 xiǎohán | 小寒 shōkan | 소한 (小寒) sohan | Tiểu hàn | Jan 6 | |
| 300° | major cold | 大寒 dàhán | 大寒 daikan | 대한 (大寒) daehan | Đại hàn | Jan 20 |
From the beginning of spring 立春 to the great cold 大寒。Each of the solar terms has about 15 days more or less. Due to the speed of the earth along the ecliptic (黄道）varies depending on its distance from the sun (because of Earth's orbital eccentricity), some solar terms are longer than the others.
Solar Term and Month Command.
Month command is determined by the day in the Solar Terms when a person is born. This poem from YHZP tells which element has the command by referring to the Solar Terms.
立春念三丙火用，餘日甲木旺提纲。
Lichun, twenty three Bing fire is used, the rest of the days Jia wood is prosperous and the key factor.
惊蛰乙木未用事，春分乙木正相当。
Jing Zhe, Yi wood yet to take command, ChunFen, Yi wood is rightly taking charge.
清明乙木十日管，后来八日癸水洋；穀雨前三戊土盛，其中土旺要消详。
QingMing, Yi wood takes charge for 10 days, after that Gui water becomes ocean for 8 days; GuYu first three Wu earth is prosperous, the things inside the prosperous has to be analyzed.
立夏又伏戊土取；小满过午丙火光。
LiXia, again is taken by Wu earth; XiaoMan after Wu (horse) Bing fire shines.
芒种己土相当好，中停七日土高张；夏至阴生阳极利，丙丁火旺有土张。
MangZhong, Ji earth is really good, stops at the middle 7 days earth rises and expands; XiaZhi is when Yin is born and Yang is at its extreme, Bing and Ding fire are prosperous and earth expands.
小暑十日丁火旺，后来三日乙木芳，己土三日威风盛；大暑己土十日黄。
XiaoShu, ten days Ding fire is prosperous, after that for 3 days Yi wood is frangrant, Ji earth for 3 days is impressively prosperous; DaShu Ji earth is 10 days yellow.
立秋十日壬水涨；处暑十五庚金良。
LiQiu, 10 days Ren water rises; ChuShu, 15 Geng metal is good.
白露七日庚金旺，八日辛兮祇独行。
BaiLu, 7 days Geng metal prospers, 8 days Xin only walks alone.
寒露七日辛金管，八日丁火又水降；霜降己土十五日，其中杂气取无妨。
HanLu, 7 days Xin metal take charge, 8 days Ding fire and water surrenders; ShuangJiang, Ji earth fifteen days, the mixed Qi inside might as well be taken.
立冬七日癸水旺，壬水八日更流忙；小雪七日壬水急，八日甲木又芬芳。
LiDong 7 days Gui water prospers, Ren water 8 days gets busy flowing; XiaoXue 7 days Ren water is in hurry, 8 days Jia becomes fragrant again.
大雪七日壬水管；冬至癸水更潺汪。
DaXue 7 days Ren water takes charge; DongZhi Gui water's sounds of flowing accumulate.
小寒七日癸水养，八日辛金丑库藏；大寒十日己土胜，术者精研仔细详。
XiaoHan 7 days Gui water nourishes, 8 days Xin metal is stored inside storage; DaHan 10 days Ji earth wins, those who practice the art should research in great detail.
There are other versions of this poem. I do not know how they are derived and which is more accurate. At the moment I'm using this YHZP version and it seems pretty accurate to me.
Transformation and change of month command
1. Protrudes in Heaven.
2. Interaction between the branches.
If there are 2 moving Qi from a particular Month EB, one chart will have 2 Commanders. Whether this is auspicious or not will be analyzed from the basics of Follow Use 顺用 or Against Use 逆用.
When there's no change or transformation involved, the original commander dictates by the solar terms remains the commander.
Month command is the YongShen
The final month commander (after transformation or the original) is the YongShen 用神 (refer ZPZQ), which is an abbreviation of the term 用事之神 YongShiZhiShen (refer DTS) - The God that takes command. The literal translation by modern English bazi 'Masters' - Useful God is misleading because it does not reflect the true identity of this entity.
However, according to the theories in ZPZQPZ written by Master Xu LeWu who distorted Master Shen XiaoZan works - ZPZQ, YongShen is defined as the most needed entity in a chart. Master Xu's works then give rise to contemporary / modern Bazi which deviates quite a distance from the classics (in my opinion), especially on subject of YongShen.
The use of Month Command, aka YongShen
BaZi is made of layers, each of the layers is analyzed according to the concept of TiYong 体用 - Form and Use.
The first layer takes DM as the Form, and month commander as Use.
The second layer takes the month commander Ti - Form, and then seek for the Yong - Use, according to the concept of ShunYong 'Follow Use' or NiYong 'Against Use'. and the Minister God could be found and the structure can be established.
The third layer takes the structure that is made of DM, Month command and Minister God as Form, other entities in the chart to use.
The fourth layer takes the chart as a whole as the Form, DaYun etc as Use.
In summary, determining the correct Month command is arguably the most important step in analyzing Bazi, which is why the month command is often called 提纲 - the Key Point in almost all literature. It is the focal point of classical Bazi analysis.
Reference:
1. SMTH -论人元司事 Discuss Human Origin taking charge
2. DTS -月令 Month Command
3. ZPZQ - A few chapters about Yong Shen
4. SFTK - 静动说 The saying about movement and stillness
5. The book of rites 礼记 - Month Command
6. Wikipedia - Solar Terms
7. YHZP - the poem about month command.
8. DTS - 体用 Form and Use
For those who are interested to read more about YueLing on LiJi, here's the link
Note: Thanks to duesouth for pointing out the correct name - The book of rites.
