Poonool, technically known as "Yajnasoothram" or "Yajnopaveetham" is worn by the Braahmanan and certain other communities, around the trunk of their body over the left shoulder and under the right arm, and represents clothing. In early times, clothing was of two kinds, "Anthareeyam", a piece of covering cloth, and "Uthareeyam", a cloth or hide to cover the chest to keep the lungs warm and strong during Vedam recital. Poonool got the name Yajnopaveetham as it is to be specially worn during sacred rituals such as yajnam. Yajnam means Vishnu, while Upaveetham connotes a covering, and thus, together the word could mean a cover for the god that dwells in the heart. Since the soul is supposed to dwell in the heart, Upaveetham got the other name , Soothram.

Poonool is made of cotton thread or cloth or of Kusa grass wound around three times with the ends formed into a knot. There are different versions as to what the three represent - Sat-Chit-Aanandam; Sathwa-Rajas-Thamo Gunams; Brahmaavu-Vishnu- Maheswaran; Jnaana-Itchha-Kriyas; Bhoor-Bhuva-Swar Lokams, etc. Each of the three has nine strands representing nine Devans : 1. Pranavam (Omkaaram), 2. Agni, 3. Naagam, 4. Soman, 5. Pithrukkal, 6. Prajaapathi, 7. Vasu, 8. Yaman, and 9. the rest of the Devathas.

Wearers of Yajnopaveetham shall have control over their mind, word and body. It is worn to make one eligible to perform sacred rituals, for improving Brahma-Thejas, to receive God's blessings through destruction of all troubles, and to enable one to perform all the daily rites and rituals prescribed in the Sruthi and Smrthi texts. It is to be worn while serving teachers, the elderly and guests, and while performing Homam, Japam, consumption of food, Aachamanam, Vedam-learning, etc. which have led to wearing it all the time.

In olden times, women also wore Yajnasoothram. The husband brings her to the Homasaala with Poonool worn around her. During later periods, wearing of Poonool by women was considered unnecessary perhaps because unmarried women were a rarity in ancient Hindu families, and the husband and wife were considered as one entity, and the Poonool worn by the husband was considered to represent that of the wife too. This might later have led to the present stricture that women shall not wear Poonool.

Boys begin wearing Poonool with Upanayanam, which is the initiation into Brahmacharyam. It is made on an auspicious day and the maker is clean (Sudhham), and in the prescribed manner with "Pavithram" knot representing Brahmam, and symbolises the three "Gunams" that emerge from, and then merge with Brahmam. Though no Manthram is needed, first, a prayer is offered to Ganapathy and then Gaayathri may be chanted while making Poonool. However, the prescribed Manthram is to be chanted while wearing a new Poonool. It is worn over the left shoulder and under the right arm, reaching down almost to the right hip.

Poonool is not to be touched with unclean hands. Namboothiris hang the lower part of it after removing through the right arm, over the right ear lobe while in the urinal or latrine. It prevents polluting the Poonool both literally and also due to the belief that Braahmanan's right ear is blessed with the spirit of the sacred "Ganga", thereby preventing pollution. Poonool is to be reversed to the right shoulder and under the left arm during some ceremonies like "Sraaddham", "Pindam", etc., and is called "Idathidal" (wearing towards left).

A broken and mended or lost Poonool should be replaced at the earliest opportunity. It is to be worn while in an open space facing east and looking at the sun, accompanied by the chanting of prescribed Manthrams, after the morning bath and Sandhyaavandanam. The old Poonool is broken, removed, tied around a small stone and thrown into the tank. This may be followed by "Upastthaanam".