The thread is given at the upanayana, the rite that takes a boy to a teacher and makes him dvija, twice-born, the second birth being the one into study. The domestic ritual manuals, the Grihya Sutras, set out the ceremony in detail and prescribe different ages for the three upper varnas, commonly the eighth year for a brahmana, the eleventh for a kshatriya and the twelfth for a vaishya, with latest permissible ages beyond which the omission was held to disqualify.
The cord itself is made of three strands twisted together, each strand in the fuller prescriptions consisting of nine threads, knotted with a particular knot called the brahmagranthi. The material differs by varna in the older texts, cotton, hemp or wool. Interpretations of the three strands are offered in numbers: the three Vedas, the three debts owed to sages, ancestors and gods, the three qualities of nature, the three utterances of the Gayatri. The texts do not choose between them.
At the same rite the boy receives the Gayatri verse from his teacher and takes up the obligations of daily recitation. The thread is a token of that, and traditional teaching insists that the token without the recitation is empty, a point made pointedly and often within the tradition itself.