Bip39 Seed To Private Key. From this position forward the seed can be penned on paper or spoken about telephone and this opened up new, Bodily means of backing up the seed (several paper copies in different spots, long lasting copies on steel plates with further security from hearth/flood etc.). https://financeauto26825.wikififfi.com/1111889/the_anatomy_of_a_bip39_phrase_entropy_phrases_and_security