She’d been born and raised on a planet. This was a rare thing, in the Culture; even rarer than being born on a ship, and that was pretty rare in itself. Things had been like this for millennia. Anyway, she ascribed whatever eccentricities she was prone to displaying to that oddity of birthright. She’d spent more time on Os than anywhere else – spent centuries living on them – but still she couldn’t help but think of planets as normal and Orbitals as somehow aberrant, for all that the artificial worlds far outnumbered the number of naturally inhabitable ones.

In the end it was kind of unarguable that planets were natural and Orbitals were artificial, though she supposed that, when you thought about it, it was really no more natural to live on the outside of a huge, skein-warping sphere of rock, held to its surface – like the atmosphere – by nothing more than gravity than it was to live on the inside of an O, held there by spin, with the atmosphere held down by the same movement and stopped from spilling off the edges by retainment walls of diamond film and fate knew what exoticism of material and field.

She drifted in and out of her snooze, and, while still dozily waking up, asked her ancient pen terminal whether anybody had ever built a planet, or discovered a natural Orbital. Yes to the former, though rarely, and not for aeons. A straight no to the latter.

“There you are,” she told Yoawin as she got her to stand and busied herself re-setting the animal’s bridle. “Planets not guaranteed totally natural after all.”

The aphore snorted.

That night, at the start of the hills – maybe a kilometre in and a hundred metres up, by a dry water bowl – Tefwe slept under the stars and the bright, lit band of the Orbital’s far side.

That was the really unnatural sign of an O, she supposed. You might not notice the inside-out curvature at any time, or come to an access tube that would let you drop the hundred metres or whatever to the world’s space-exposed under-surface, or see the sea or the clouds held ruler-straight against an edge wall, but, come the night, there would be the sure and certain sign that you really were pinned by your rotating frame of reference to the inside of a bracelet ten million klicks across: the far side, shining in its daytime while the side you were on had its back turned to whatever sun the whole O circled.

Well, unless the clouds on your side of the Orbital were really thick, she supposed, and fell asleep.