The Egyptian crux ansata, or looped tau-cross. This magic knot or cross, known as Nem Ankh, the key of life, was often used in the iconography of opposites. The loop over the tau-cross could stand for the Sun, for Heaven and Eath as the macrocsm and for man as the microcosm. It is generally interpreted as a symbol which expresses the reconciliation of opposites of the integration of active and passive qualities. This is amply confirmed by the fact that, when recumbent, the ankh symbolizes both male and female sexual attributes in precisely the same way as the very realistic Hindo depiction of a Hermaphrodite standing on a Lotus flower. Champdor gives a more traditional interpretation as:the symbol of the millions of years of the life to come. The loop is the perfect symbol of what has neither beginning nor end and stands for the soul which is eternal because it has sprung from the spiritual essence of the gods. The cross erpresents the state of trance in which te neophyte struggled, or, more precisely, the state of death, the crucifixion of the chosen victim, and in some temples the priests used to lay the neophyte on a bed shaped like the cross... The possessor of the geometric key to the hidden mysteries, of which the symbol was this very looped cross, was able to open the gates of the Kingdom of the Dead and penetrate the hidden meaning of eternal life. Cham, p.22
Gods, kinks and Isis (almost invariably) are depicted holding the ankh to show that they command the powers of life and death and that they are immortal. The dead also carry it at the time their souls are weighed or when they are aboard the Boat of the Sun God, as a sign that they seek this same immortality from the gods. Furthermore the ankh symbolized the spring from which flowed divine virtues and the elixir of immortality. Therefore to hold the ankh was to drink from that well. It was sometimes held upside down by the loop - especially in funeral rites when it suggested the shape of a key and in reality was the key which opened the gateway of the tomb into the Fields of Aalu, the realm of eternity. Sometimes the ankh is placed on the forehead, between the eyes, and then it symbolizes the duty of the adept to keep secret the mystery into which he has been initiated - it is the key which locks these secrets away from the uninitiated. Blessed by the supreme vision, endowed with clairvoyance to pierce the veil of the beyond, he cannot attemot to reveal the mystery without losing it for ever.
The ankh is often set in the same category as the Girdle of Isis, as a symbol of eternity. This is not because its straight lines may be lengthened in the imagination to infinity, but because they converge upon and meet in a closed loop. This loop symbolizes the inexhaustible essence of the life force identified with isis, from whom life flows in all its forms. it is therefore carried by all those who wish to share her life. Hence the ankh may be identified with the Tree of Life, with its trunk and foliage.
The significance of the Girdle of Isis is far more complex. Like ropework or plaited hair round the arms and the loop of the cross, it infuses the concept of life and of immortality with the concept of the knots which tie down mortal life on Earth and which must be unravelled to enjoy immortality. `Free your bonds,' says The Egyptian Book of the Dead, `untie the knots of Nephthys.' And again: `Shining are those who carry the girdle. Oh! Bearers of the Girdle.' The same meaning is conveyed by the Tibetan Buddhist book called The Book of the Untying of the Knots. While the plain looped cross symbolizes divine immortality, sought orattained, the Girdle of Isis makes clear the conditions under which that immortality is obtained - by the untying of knots - dénouement in the true sense of the word.
A Dictionary of Symbols, Jean Chevalier and Alain Gheerbrant, translated from the French by John Buchan-Brown, Blackwell, 1994 (the French edition was originally published by Éditions Robert Laffront S.A. in 1969, 2nd ed. 1982)