The triple world is the oldest continuously used map in Indian religion, and its endurance owes much to its refusal to stay literal. In the Vedic hymns the three levels are concrete: the earth underfoot, the atmosphere where wind and storm and the middle gods operate, and the bright sky beyond, each with its resident powers. The three ritual utterances bhur, bhuvah, svah, recited daily for some three thousand years in the Gayatri practice, seal that geography into liturgy. But already in the Upanishads the levels begin to bend inward, correlated with states of the person, waking, dream and deep sleep, and with the parts of the sacred syllable Om, so that ascending the worlds and deepening into oneself become the same journey described twice.
Hindu cosmology later multiplied the map, stacking seven upper worlds and seven nether ones, yet kept the ancient triad as the core that dissolves and is renewed with each day of Brahma. The scheme also organized practice: the three debts, the three fires, the threefold division of ends. A map this reusable stops being a picture of the universe and becomes a grammar for it, a way of saying that reality comes in levels, that the levels are ordered, and that beings move among them, claims every Indian tradition wanted to make, however differently each drew the details.