丁 Dīng — the Yin Fire Day Master

What the classics read it as

Yin Fire. Lamplight — the small managed flame that has to be fed, and that lights one room at a time.

「丁火柔中，內性昭融」. Ding fire is soft on the outside and its inner nature is clear and warm. Where 丙 arrives whether or not it is wanted, 丁 is brought. Somebody carried it in, somebody keeps it fuelled, and it goes out if that stops. This is the only Day Master in the set whose survival is openly conditional, and the classics do not present that as a flaw.

What it loves: 「抱乙而孝，合壬而忠」 — holding 乙 it is filial, joined to 壬 it is loyal. 乙 is grass and 甲 is timber, and both are fuel; the practical reading is that this stem needs a supply and does its best work when the supply is steady rather than large. 「如有嫡母，可秋可冬」 — with a proper source it can take autumn or winter, the two seasons that finish most fires.

What it fears: running out. Not water, which the classics barely raise for 丁, and not metal, which it works on. Depletion.

The character: 「旺而不烈，衰而不窮」 — strong without violence, weak without being finished. It is the steadiest of the ten and the least visible. A lamp illuminates a table, and what it does not reach stays dark, and that limitation is the same property as the precision.

The cost: this Day Master gets its results one at a time and cannot be spent broadly without going out. Choosing which room to be in is not a preference here. It is the whole method.