封 sets 圭 (guī) beside 寸 (cùn): ⿰圭寸. The 寸 on the right is a hand with a wrist-pulse marked, the standard graph for a deliberate manual act. The left side traces back through doubled 土 (tǔ, earth): the seal and bronze forms show a hand patting soil into a raised mound. The character records a single physical gesture, heaping earth by hand to mark and close a boundary.

The oldest sense is territorial. To 封 a piece of land was to raise a mound of packed earth along its edge, often planting a tree on top, so that the line of ownership stood visible on the ground. From that mound came the political sense: a ruler who granted territory was said to 封 a lord, fixing the borders of a fief and the rank that came with it. 分封 (fēnfēng), the carving of the Zhōu realm into hereditary domains, is built on exactly this image.

The everyday modern sense, to seal shut, is the same gesture turned inward. Where the feudal lord closed off a border with earth, an ordinary writer closes off a letter with a flap of paper. 封 covers both the act (to seal, to block) and the object that holds the sealing (信封, envelope; one 封 of mail). The measure-word use, 一封信 (yì fēng xìn, one letter), preserves the idea that each sealed packet is a single closed unit.

Japanese keeps the graph as 封 (fū / hō), used in 封筒 (envelope) and 封建 (feudal). Korean reads it 봉 (bong) in 봉투 (envelope) and 밀봉 (hermetic seal). Across all three, the thread holds: a hand closing something off so it stays shut.

The feudal sense is alive in formal and historical Chinese. To 封 someone is to grant them a title, a fief, or a rank from above. 分封制 (fēnfēng zhì) names the Zhōu system of parcelling out land to kinsmen and allies; 册封 (cèfēng) is the formal investiture of a noble or consort by imperial decree. The grammar still carries the old direction: rank flows downward from the one who 封s to the one who is 封ed.

See a lord kneeling at the edge of his new land at dusk. He scoops up a double handful of damp earth, 圭 of soil, and pats it into a low mound along the border with his own hand, 寸. The cool grit packs under his palm; he presses until the line of ownership stands raised and unmistakable. That heap of earth, closed by hand, is 封.

Now shrink the gesture. The same hand that mounded a border later licks a flap and presses it down over a letter: 信封, the envelope. The same closing motion shuts a road (封闭), locks down a city (封锁), and squeezes a jar airtight (密封). Whether the thing sealed is a kingdom's edge or a single sheet of paper, 封 is always one hand pressing a boundary shut.

Hold the two extremes together and the character locks in: a fief granted from above and an envelope licked at the kitchen table are the same verb. To 封 is to close something off so it stays closed, then hand it on.