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The Vitarka mudrā “mudra of discussion” is the gesture of discussion and transmission of Buddhist teaching. It is done by joining the tips of the thumb and the index together, and keeping the other fingers straight very much like the abhaya and varada mudrās but with the thumbs touching the index fingers. … Vitarka mudrā is also known as Prajñāliṅganabhinaya and Vyākhyāna mudrā (“mudra of explanation”).

when the discourse is just right