For three centuries the Aethyrs were a table in a manuscript. What made them a working system was Aleister Crowley's decision to go through all thirty in sequence.
He had scried some of the lower ones in Mexico in 1900. In 1909 he took the remainder in North Africa, travelling in the desert with the poet Victor Neuburg, working from the thirtieth upward toward the first, reciting the call and dictating what he saw and heard while Neuburg wrote. The record was published as The Vision and the Voice and is the fullest such document in modern Western esotericism, whatever one concludes about its status.
The working's centre is the tenth Aethyr, ZAX, which he identified with the Abyss of the kabbalistic tree and where he described an encounter with a being called Choronzon, conducted with Neuburg inside a protective circle and the encounter taking place in a triangle drawn in the sand. The account of that operation is the passage most often reproduced.
Out of it Crowley built a map of initiation. The upper Aethyrs are read as corresponding to the highest grades of his system, the crossing of the Abyss is placed at ZAX, and the sequence becomes a description of stages a practitioner is expected to pass through rather than a survey of places.
That is worth naming precisely. The Aethyrs as a scheme are Elizabethan; their use as a ladder of attainment is Crowley's, and the two should not be read back into one another.