The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

The ring had two keys now, brass and copper. Beto shook it so it jingled.

The spark rolled down the lane behind the print shop, and the lane became a street of shops. These shops sold mirrors, locks, puzzle boxes and paper wheels. Every sign was written backwards.

Inkling read one in a shop window. It said MIRROR LANE the right way round.

One shop had no sign at all. A note was pinned to its door, addressed to the company.

Danya read it out, letter by letter. "N, V, V, G. N, V. Y, V, H, R, W, V."

The whole note said: NVVG NV YVHRWV GSV TIVVM WLLI.

"It's a code," said Inkling. "I can name it. It's a secret message."

"We know that much," said Danya. "How do we read it?"

"I could take it apart," said Beto. "Last term the club had a code wheel. You slide every letter back three."

He worked it out on his fingers. N went back three to K. V went back three to S. G went back three to D.

"K, S, S, D," said Beto. "Kssd."

"That isn't a word," said Inkling.

A swan glided out of the shop next door with a hand mirror.

"Fold the alphabet in half," said Mira. "This lock is a mirror, not a slide. Mirror it and read again."

Beto put the wheel down. "So the wheel was the wrong tool," he said. "Good to know."

An owl came out of a doorway with a sieve full of paper letters.

"Which letter turns up most?" asked Sift. Then she shut her eyes, as if she had all day.

Inkling tallied on the back of her hand. She went slowly this time.

"V," said Inkling. "Eight times. Nothing else comes close."

She looked at the tally. "We did letters in class," she said. "E is the commonest letter in English. So if this note is English, V might be E."

"Might," said Sift, and opened one eye.

Try the Packet now, if you have it. Then read on.

Mira wrote the alphabet in a line. Then she folded the paper in half, so A sat on top of Z, and handed it to Danya without a word.

Danya ran a finger down the fold. A sat on Z. B sat on Y. C sat on X.

"E is the fifth letter," he said. "And V is the fifth from the end."

He looked up. "They're twins across the fold. So V isn't just probably E. It is E."

"Every letter has a twin," said Mira, pleased.

A fox in a carnival mask leaned on the green-painted shop at the end of the lane.

"One swap, the same every time," said Mask. "Keep the key on one page."

Beto took the folded paper. He had used the wrong wheel; he wanted this one. He wrote the key out properly, A over Z, B over Y, all the way down. Then he read the note again, letter by letter.

"NVVG is MEET," he said. "NV is ME. YVHRWV is BESIDE. GSV is THE. TIVVM is GREEN. WLLI is DOOR."

"Meet me beside the green door," said Inkling.

"How do we know it's right?" asked Danya. "It's a sentence. And there's only one green door on this lane."

They looked. There was. Its old top hinge had been replaced with a new one, set neatly into the worn wood, and beside it somebody had scratched a tiny hammer.

The cipher-wright was waiting behind the green door. She was a small woman with ink on every finger.

"Most people use the wrong wheel and give up," she said. "You dropped it and counted." She gave them a silver key from Mirror Lane.

The spark rolled back up the lane, and the lane was just the lane behind the print shop.

Back on the Green

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

Inkling wrote her name in Atbash on her lunch box. RMPORMT. Beto counted the letters and worked it out before the bell.