正 zhèng = 一 (a line, the target) + 止 (zhǐ, foot). The lower element 止 is the ancient pictograph of a foot, and the stroke above it marks a place to reach. The oracle-bone form shows a foot advancing on a walled town or a fixed mark; the early sense is to march straight toward a target and halt squarely on it. From "to head straight at the mark and stop on it" came the whole field of meaning: straight, upright, correct, exact.

Four senses branch from the foot on the line:

1. Straight, upright, correct. Hitting the mark without deviation: 正确 zhèngquè (correct), 公正 gōngzhèng (just; fair), 端正 duānzhèng (upright in form and conduct).
2. Exactly, precisely. Squarely on the spot: 正好 zhènghǎo (just right), 正是 zhèngshì (precisely is).
3. Principal, main, regular. The central one as opposed to the deputy or the deviant: 正门 zhèngmén (main gate), 正常 zhèngcháng (normal).
4. In the midst of (the ongoing). The action squarely underway right now: 正在 zhèngzài (in the process of), as in 我正在吃饭 "I'm right in the middle of eating."

The thread is the mark hit dead center. Whether the target is moral (justice), factual (correctness), spatial (the main entrance), or temporal (the present instant), 正 is the foot landing exactly where it should, with nothing crooked or off to the side.

One of the first jobs a learner meets 正 doing is marking the present continuous. 正在 zhèngzài placed before a verb says the action is squarely underway at this moment: 我正在吃饭 "I'm (right in the middle of) eating," 他们正在开会 "they're in a meeting right now." The 正 supplies "right at this point," the 在 supplies "in the act of," and a final 呢 ne often rounds it off: 正在下雨呢 "it's raining right now."

The aspectual 正 is the temporal cousin of the spatial one. Just as 正 marks the foot landing dead center on a target, 正在 marks the action caught dead center in its course, neither before it has begun nor after it has finished, but exactly in the midst. To go deeper on continuous aspect and the 在 / 着 / 呢 system, see the 在 entry.

See a foot, 止, stepping up to a line, 一, and stopping exactly on it: dead center, no swerve, no overshoot. That foot on the mark is the whole character. Everything 正 means is the foot landing where it should.

On a question, that is correctness (正确). On a scale of conduct, it is justice (公正). On a measurement, it is the exact fit (正好). On a building, it is the main gate rather than the side door (正门). And on the clock of an action, it is the present instant, the deed caught squarely in progress (正在). Whenever you meet 正, ask what the target is, and you will find the foot resting on it.