The knot in the sacred thread is called as Brahma-Granthi. Granthi refers to a knot. This Brahma-Granthi contains a set of three knots each representing Lord Brahma, Vishnu and Maheshwara. Shiva would be usually depicted in the pictures as wearing the Servant Vasuki as the sacred thread.
Even we can see in the pictures of Lord Vinayaka, Muruga and other male deities, wearing the sacred thread in their bodies. Mahavishnu in his various avatars used to wear the sacred thread. In the Vamana avatar, Lord Vamana is depicted as a small young boy, who contains a cheerful and handsome face, carries an umbrella and a Kamadala in his hands similar to a Rishi, and he wears the sacred thread in his body, which beautifies his appearance. Even some pious demons like Prahalada, Vibhishana and Namuchi used to wear the holy sacred thread permanently on their bodies, and with that, they would appear as Brahmanic-demons.
During the time of Vinayaka Chathurti festival, after buying the clay Vinayaka from the road side shops, we would prepare a sacred thread for Lord Vinayaka using cotton, in order to make it as attractive. In ancient Vedic times, both Brahmins and Non Brahmins used to wear the sacred thread, in order to chant the Vedic Mantras. Even the demon Ravana used to wear the sacred thread permanently, since he is an expert in chanting the Vedic Mantras.
Sacred thread is considered as very holy for Brahmins, and it is as much sacred as the holy Mangalsutra. It must never be removed from the bodies of the Brahmins, and it also acts as a protective shield like Talisman (Thayathu) during the times of hardships. But nowadays, without knowing the significance of sacred thread, even some of the Brahmins, are not reciting the Vedic Mantras, and they are also removing the sacred thread from their body and hanging it in the coat stands, and they used to wear it only occasionally, especially during the times of marriages and death ceremonies.