My own room held no promise. I knew every inch of it and it knew me; we were dull companions now. Instead, I pushed open the door of the guest room. The blank-faced wardrobe and bare dressing table paid lip service to the idea that you could brush your hair and get dressed here, but somehow you knew that behind their doors and drawer fronts they were empty. The bed, its sheets and blankets tightly tucked in and smoothed down, was uninviting. The thin pillows looked as though they had had the life drained out of them. It was always called the guest room, but we never had guests. It was where my mother slept.

Perplexed, I backed out of the room and stood on the landing.

This was it. The rite of passage. Staying home alone. I was joining the ranks of the grown-up children: Tomorrow I would be able to say, in the playground, "Last night I didn't go to a sitter. I stayed home by myself." The other girls would be wide-eyed. For so long I had wanted this, and now that it was here, I didn't know what to make of it. I'd expected that I would expand to fit the experience automatically, that I would get my first glimpse of the person I was destined to be. I'd expected the world to give up its childlike and familiar appearance to show me its secret, adult side. Instead, cloaked in my new independence, I felt younger than ever. Was there something wrong with me? Would I ever find out how to grow up?

I toyed with the idea of going round to Mrs. Robb's. But no. There was a better place. I crawled under my father's bed.

The space between the floor and the bed frame had shrunk since I was last there. Hard against one shoulder was the holiday suitcase, as gray in daylight as it was here in the dark. It held all our summer paraphernalia: sunglasses, spare film for the camera, the swimming costume that my mother never wore but never threw away. On the other side was a cardboard box. My fingers fumbled with the corrugated flaps, found a way in, and rummaged. The tangled skein of Christmas-tree lights. Feathers covering the skirt of the tree angel. The last time I was under this bed I had believed in Father Christmas. Now I didn't. Was that a kind of growing up?

Wriggling out from under the bed, I dislodged an old biscuit tin. There it was, half sticking out from under the frill of the valance. I remembered the tin-it had been there forever. A picture of Scottish crags and firs on a lid too tight to open. Absently I tried the lid. It gave way so easily under my older, stronger fingers that I felt a pang of shock. Inside was Father's passport and various, differently sized pieces of paper. Forms, part printed, part handwritten. Here and there a signature.

For me, to see is to read. It has always been that way. I flicked through the documents. My parents' marriage certificate. Their birth certificates. My own birth certificate. Red print on cream paper. My father's signature. I refolded it carefully, put it with the other forms I'd already read, and passed on to the next. It was identical. I was puzzled. Why would I have two birth certificates?

Then I saw it. Same father, same mother, same date of birth, same place of birth, different name.

What happened to me in that moment? Inside my head everything came to pieces and came back together differently, in one of those kaleidoscopic reorganizations the brain is capable of.

I had a twin.

Ignoring the tumult in my head, my curious fingers unfolded a second piece of paper.

A death certificate.

My twin was dead.

Now I knew what it was that had stained me. Though I was stupefied by the discovery, I was not surprised. For there had always been a feeling. The knowledge, too familiar to have ever needed words, that there was something. An altered quality in the air to my right. A coagulation of light. Something peculiar to me that set empty space vibrating. My pale shadow.

Pressing my hands to my right side, I bowed my head, nose almost to shoulder. It was an old gesture, one that had always come to me in pain, in perplexity, under duress of any kind. Too familiar to be pondered until now, my discovery revealed its meaning. I was looking for my twin. Where she should have been. By my side.

When I saw the two pieces of paper, and when the world had recovered itself enough to start turning again on its slow axis, I thought, So that's it. Loss. Sorrow. Loneliness. There was a feeling that had kept me apart from other people-and kept me company-all my life, and now that I had found the certificates, I knew what the feeling was. My sister.

After a long time there came the sound of the kitchen door opening downstairs. Pins and needles in my calves, I went as far as the landing, and Mrs. Robb appeared at the bottom of the stairs.

"Is everything all right, Margaret?"

Yes.

"Have you got everything you need?"

"Yes."

"Well, come round if you need to."

"All right."

"They won't be long now, your mum and dad."

She left.

I returned the documents to the tin and put the tin back under the bed. I left the bedroom, closing the door behind me. In front of the bathroom mirror I felt the shock of contact as my eyes locked together with the eyes of another. My face tingled under her gaze. I could feel the bones under my skin.

Later, my parents' steps on the stairs.

I opened the door, and on the landing Father gave me a hug.

"Well done," he said. "Good marks all round."

Mother looked pale and tired. Going out would have started one of her headaches.

"Yes," she said. "Good girl."

"And so, how was it, sweetheart? Being home on your own?"

"It was fine."

"Thought it would be," he said. And then, unable to stop himself, he gave me another hug, a happy, two-armed affair, and kissed the top of my head. "Time for bed. And don't read too long."

"I won't."

Later I heard my parents going about the business of getting ready for bed. Father opening the medicine cupboard to find Mother's pills, filling a glass with water. His voice saying, as it so frequently did, "You'll feel better after a good night's sleep." Then the door of the guest room closed. A few moments later the bed creaked in the other room, and I heard my father's light click off.

I knew about twins. A cell that should ordinarily become one person inexplicably becomes two identical people instead.

I was a twin.

My twin was dead.

What did that make me now?

Under the covers I pressed my hand against the silver-pink crescent on my torso. The shadow my sister had left behind. Like an archaeologist of the flesh, I explored my body for evidence of its ancient history. I was as cold as a corpse.

With the letter still in my hand, I left the shop and went upstairs to my flat. The staircase narrowed at each of the three stories of books. As I went, turning out lights behind me, I began to prepare phrases for a polite letter of refusal. I was, I could tell Miss Winter, the wrong kind of biographer. I had no interest in contemporary writing. I had read none of Miss Winter's books. I was at home in libraries and archives and had never interviewed a living writer in my life. I was more at ease with dead people and was, if the truth be told, nervous of the living.

It probably wasn't necessary to put that last bit in the letter. I couldn't be bothered to make a meal. A cup of cocoa would do. Waiting for the milk to heat, I looked out of the window. In the night glass was a face so pale you could see the blackness of the sky through it. We pressed cheek to cold, glassy cheek. If you had seen us, you would have known that were it not for this glass, there was really nothing to tell us apart.