Outside the enclosure, at the four angles, but at unequal distances are four great Labab chorten, that is to say chorten which carry on their four sides four staircases; they commemorate, according to the Indo-Tibetan tradition, the ascent of Buddha to the Tusita paradise, when he went to preach the gospel there to his mother, already ascended there after his earthly death. - Giuseppi Tucci: Secrets of Tibet (September 19, 1933)