They were sitting in a carousel suspended beneath a vast carbon-tubed structure sculpted in the image of a web tree; the thousands of viewing carousels dangled like fruit from the canopy and were multifariously connected by a secondary web of delicate, swaying cable bridges. The view beneath and to either side was of a series of great steps of stone dotted with vegetation and moving figures; it was very like looking at an ancient amphitheatre which had been lifted from the horizontal to the vertical and each of whose seat levels was able to rotate independently. The moving figures were ysner-mistretl combinations; the ysners were the huge two-legged flightless (and almost brainless) birds doing the running while their thinking was done by the mistretl jockey each carried on its back. Mistretls were tiny and almost helpless but brainy simians and the combination of one of them per ysner was a naturally occurring one from a planet in the Lower Leaf Spiral.

Ysner-mistretl races had been a part of life on Tier for millennia, and running them on a giant mandala two kilometres across composed of steps or levels all rotating at different speeds had been traditional for most of that time. The huge slowly turning race-course looked a little like Tier itself, which took its name from its shape.

Tier was a stepped habitat; its nine levels all revolved at the same speed, but that meant that the outer tiers possessed greater apparent gravity than those nearer the centre. The levels themselves were sectioned into compartments up to hundreds of kilometres long and filled with atmospheres of different types and held at different temperatures, while a stunningly complicated and dazzlingly beautiful array of mirrors and mirrorfields situated within the staggered cone of the world’s axis provided amounts of sunlight precisely timed, attenuated and where necessary altered in wavelength to mimic the conditions on a hundred different worlds for a hundred different intelligent species.