通 tōng is a pictophonetic compound: the walk-movement radical 辶 (chuò, a foot moving along a road, abbreviated from 辵 which shows a foot and a crossroads) carries the meaning, while 甬 yǒng supplies the sound. The 辶 places 通 squarely in the family of motion characters, the same radical that drives 道 (the Way), 过 (to pass through), 近 (near), and 远 (far). Where those characters measure distance or name the path, 通 names a different quality of the path: that it is open, that movement along it meets no obstacle.

The phonetic 甬 is itself worth noticing. In its own right it named a covered walkway or a lane, a passage with walls on either side that channels movement straight through. So even the sound-bearing half of 通 carries an echo of corridors and through-routes. The combined character means to go through, to pass without being stopped, to get from one side to the other.

The Shuōwén Jiězì glosses 通 as 達也 (dá yě), to reach through, to arrive by passing through. From this root sense of unobstructed passage every later meaning radiates. A road that is 通 is open to traffic. A message that 通 has gotten through to its recipient. A person who is 通 in a subject has passed all the way through it and understands it thoroughly. And something that is 通 across many cases is general, common, the ordinary state that holds throughout. Passage, communication, mastery, and generality are one image seen from four sides: nothing blocks the way.

通 carries the same range across the Sinosphere. Japanese 通 (tsū / tsūjiru) gives 交通 kōtsū (traffic), 通信 tsūshin (communication), and 通 used alone for a connoisseur, someone who has passed all the way into a field. Korean 통 (tong) appears in 교통 (gyotong, traffic) and 통신 (tongsin, communication). Vietnamese thông survives in giao thông (traffic) and thông tin (information), direct borrowings of the Chinese compounds.

A foot setting off down a covered corridor that runs straight from one end to the other, with no door bolted across it and no wall built halfway. That is the picture inside 通: the walk-movement radical 辶 is the foot on the move, and the phonetic 甬 was itself a name for a walled passage that channels you straight through. Whatever the corridor leads to, the point is that you reach it. Nothing stops the way.

Hold that one image and every meaning falls out of it. A road that lets vehicles run from end to end is 通车, open to traffic; the whole crossing-and-running network is 交通. A phone call that finds its way to the person you dialed has 打通; a message that reaches its recipient is a 通知; people who open a channel of understanding between them 沟通. A person who has walked all the way through a subject down to its fine grain is 精通, a master of it. And what holds throughout, the common case running across all instances, is 普通, ordinary, or 通用, good everywhere.

The negative is just as sharp. When the corridor is jammed so tight that not even water could leak through, you have 水泄不通, the exact opposite of 通. Keep both ends in mind: the open corridor that lets everything pass, and the blockage that lets nothing. 通 is always the version where the way is clear.