It turned out that she was first, clattering up the stairs into a classroom occupied only by Sister Rule.

‘Good morning, novice.’ Sister Rule glanced up from behind the desk where she tended to settle her bulk and remain for the duration of any lesson, using her yardstick to point to the headings set out in chalk upon the board to her side.

‘Good morning, Mistress Academia.’ Nona found her seat at the back of the class.

‘Brisk out, is it?’

‘Yes, Mistress Academia.’ The curved sheet of ice that had formed across the side of Nona’s head chose that moment to fall away and shatter on the floor.

Hessa stumped in a moment later, looking surprised not to be the first.

‘What’s the convent’s most valuable treasure?’ Nona surprised herself by asking but Sister Rule always took kindly to questions and asking had become a habit.

‘The shipheart.’ No hesitation.

‘And it’s stored below Heart Hall?’

‘Yes indeed.’ Sister Rule closed the book she had been reading from and narrowed her eyes at Nona. ‘Planning on stealing it?’ She maintained the stare long enough for Nona to think she might be serious, then laughed.

The rest of Grey Class came in more or less together, Ara looking tired, Clera slumping dramatically over her desk, Jula setting her quill, scroll, and slate out with her usual precision, Darla hulking over her work and managing to spatter ink as she opened the pot. Head-girl Mally at the front. Alata and Leeni side by side, each finding the other’s fingers beneath the desk with accurate devotion. Zole took her customary seat at the middle of the room, looking serene, as if she never left the trance.

‘Today.’ Sister Rule slapped her yardstick down. ‘We will discuss the ice.’

The faintest of frowns creased Zole’s brow.

‘In Red Class we discussed the thickness of the ice, its advance, both historically and contemporaneously. And the cause – the waning of our own dying sun. Today we will address the peoples who make it their home.’ She turned her head towards Zole and gave a friendly smile. ‘Which is something Novice Zole knows far more about than anyone in the room. So perhaps she would like to tell us something of the ice-tribes?’

‘No.’ Zole’s lips barely moved but the word came out loud and clear.

Sister Rule pressed her own lips into a flat line that managed to convey surprise, amusement, and disapproval all at the same time. ‘And why might that be, novice?’

‘The ice must be experienced. You have no words in your books that capture it. Journeying the ice is the price for such knowledge.’

‘Well …’ Sister Rule rapped her yardstick against the desk. ‘We have our first observation – the tribes of the deep ice have their own codes of behaviour which can be quite at odds with our own. You would have to travel hundreds of miles east to Scithrowl or west across the sea to Durn to experience significant cultural differences in the Corridor, but a journey of less than thirty miles to the north or south will set you face to face with peoples whose ways are more alien to ours than those of any Durnishman.’ She looked around the class. ‘I’m sure you’ve been curious about your new companion and her guardian. Ask me some worthwhile questions and I’ll share what …’ her eyes flicked towards Zole ‘… meagre information I have.’

‘What do they eat?’ Nona’s hand shot up but she’d already blurted out the question.

‘Good!’ Sister Rule banged the desk. ‘To the heart of the matter. They eat fish.’

‘But how? The ice is miles thick!’ Nona had seen the walls of the Corridor and even there just behind the margins the ice lay hundreds of yards deep.

‘It is.’ Sister Rule reached for the glittering white globe on her desk with its belt of colour almost vanishingly thin, kept clear by the passing heat of the focus moon. ‘But there are places above the ocean where the sea lies open, the ice melted by warmer water upwelling from the ocean depths. In some cases the sea is accessible year round. Further north and south, where seasons rule, the ice only clears in high summer. In all cases the open water is limited to patches no more than a mile across but thick with ocean life. The ice-tribes either centre their existence on a permanent spot or range nomadically with the seasons from one temporary oasis to the next. In their travels they explore tunnel systems wherever they are to be found, looking for routes to the bedrock and any of the undercities abandoned by the Missing.’

Nona’s thoughts wandered to the stories her father told about the ice tunnels. The memories of him telling her had escaped, leaving just an impression of being bounced upon his knee, awed by the strangeness of his tales. The stories themselves she knew from her mother’s retelling of them when she was still very young.

She watched the class with eyes that saw nothing except dark, glistening tunnels worming their way ever deeper, filled with old night and new possibility. When Bray sounded and shook her from her adventures Nona was startled to find that the whole session had passed and all around her girls were rising from their chairs.

In Blade after lunch they practised with throwing stars. The novices spent the best part of two hours aiming at target boards. Sister Tallow had set up a dozen of the seven-foot boards on which man-shapes had been drawn in sufficient anatomical detail to amuse girls. The first four boards she placed just three yards in front of the throwing line, the next four six yards further back, the last another six yards out.

‘If you wish to hit an opponent further away than that you really should have brought a bow with you. Throwing stars are less encumbering and less obvious, ideal for urban situations and use inside buildings. The price paid is that they are less accurate and do less harm.’ Sister Tallow paced behind the ranks of novices lined for their turn to throw.

They threw until their arms grew tired and their hands bled from a dozen small nicks. Then they threw some more.

‘You think this is difficult?’ Sister Tallow continued her pacing, pausing here and there to correct the position or action of a novice. ‘You’ll be lucky if you ever get a real target standing still and facing you. Aim for the eyes. Anywhere on the head gives you a decent chance of a hit that will take the fight out of someone, but the eye – now that’s a guaranteed stop for anyone.’

Next to Nona, Ruli, who had proved annoyingly good with the throwing star since their first lesson, cracked her arm out and with a flip of her wrist put one into the eye of the nearest target, the point bedded in the black pupil.

‘Show-off,’ Clera said to her right. The target before Clera had three of her stars studding it, one in the forehead, one near the ribs and a third hanging about a foot from the man’s neck, having missed completely. ‘Do it again.’

Ruli sent another star spinning from her fingers, putting a point into the pupil of her target’s other eye.

‘That’s not even fair! You can’t judge where the point will land!’ Clera flung her next star and nearly missed the board entirely.

Ruli shrugged. ‘If the target were any closer and I had a spear I could practically lean forward and poke it.’

Nona threw her four stars in quick succession. All four thunked home in the head, two into the eyes.

At the end of the line Darla took her last throw and all four of them moved up to retrieve their stars before the next rank had their turn.