节 began as a bamboo joint. The traditional form 節 is a pictophonetic compound: the bamboo radical 竹 (zhú) at the top carries the meaning, and the element below supplies the sound. The original referent is the knot on a bamboo stalk, the hard ring where the cane is pinched in and from which it grows on in a fresh segment. The whole field of meaning unfolds from that one image: a node, and the section between nodes.

The simplified 节 keeps the grass radical 艹 over a single lower element, but the idea is unchanged. Think of any stalk of bamboo: it is not a smooth tube but a series of segments, each one marked off by a knot. That is what 节 names: the dividing point and the piece it divides.

Four senses branch from the bamboo node:

1. Joint, node, segment. The knot in a stalk, the joint in a limb, the link in a chain: 关节 guānjié (a joint of the body), 章节 zhāngjié (chapters and sections), 环节 huánjié (a link in a process).
2. Festival, holiday. A festival is a knot in the calendar, a marked point that divides the flowing year into seasons: 节日 jiérì (festival), 春节 Chūnjié (Spring Festival), 节气 jiéqì (a solar term).
3. To economize, to restrain. To hold a thing within its segment, to not let it run on: 节约 jiéyuē (to economize), 节省 jiéshěng (to save), 节制 jiézhì (to restrain).
4. Measure word for sections. A counter for things that come in segments, above all class periods and the cars of a train: 一节课 yī jié kè (one class period), 三节车厢 sān jié chēxiāng (three train cars).

The thread through every sense is the segment marked off by a knot. A joint divides a body, a festival divides a year, restraint divides excess from enough, and the measure word counts the pieces. Wherever something is partitioned into orderly sections, 节 is the knot between them.

A festival is to the calendar what a knot is to a stalk of bamboo: the fixed point that marks one segment off from the next. The flowing year would be a featureless tube without these knots, and 节 supplies them. 春节 Chūnjié (Spring Festival) knots the start of the lunar year, 中秋节 Zhōngqiūjié (Mid-Autumn Festival) marks the harvest moon, and the 二十四节气 èrshísì jiéqì (twenty-four solar terms) divide the agricultural year into its turning points.

The same 节 names the made-up holidays of the modern calendar too: 国庆节 Guóqìngjié (National Day), 劳动节 Láodòngjié (Labor Day), 儿童节 Értóngjié (Children's Day). Whether the festival is ancient and seasonal or recent and civic, Chinese frames it the same way, as a 节, a marked segment of time set apart from the ordinary days around it.

Because 节 means "a segment," it doubles as the measure word for things that come in discrete sections. The two everyday uses a learner meets first are class periods and train cars: 一节课 yī jié kè (one class period), 下午有三节课 "there are three periods this afternoon," and 这列火车有十二节车厢 "this train has twelve cars." It also counts batteries (一节电池 "one battery cell") and segments of a stalk or pipe.

The same node-and-segment idea runs through its compound nouns. 关节 guānjié is a joint of the body, the hinge where two bones meet. 章节 zhāngjié names the chapters and sections of a book. 环节 huánjié is a link in a chain or a stage in a process, as in 关键环节 "a key step." In every case 节 is the segment, and often the joint between segments, the same knot of bamboo seen from a different angle.

Hold a stalk of bamboo in your mind. It is not a smooth tube but a row of segments, each pinched off by a hard knot. That knot is 节, and so is the segment it marks. Once you see the bamboo, every meaning falls into place.

The knot in your knee is 关节, a joint. The knot in a book is its 章节, the chapter break. The knot in the year is the 节日, the festival that divides one season from the next. And to keep a thing from running on past its segment is to 节约, to economize. Even the measure word is the bamboo: a class period or a train car is one more segment counted off by a 节. Whenever you meet the character, find the knot, and the sense will follow.