For the first twelve days of our life, we were one person. Our father’s brains and our mother’s beauty swirled into one blessed embryo, the sole heir to the Carmichael fortune.

On the thirteenth day, we split. It was almost too late. One more day and the split would have been incomplete. Summer and I would have been conjoined twins, perhaps sharing major organs, facing a choice between a lifetime shackled together and a surgical separation that might have left us maimed.

As it was, our rupture was imperfect. We might look identical, more than most twins, but we’re mirror twins, mirror images of each other. The minute asymmetries in my sister’s face—her fuller right cheek, her higher right cheekbone—are reproduced in my face on the left side. Other people can’t see the difference, but when I look in the mirror, I don’t see myself. I see Summer.

When we were six years old, Dad took a sabbatical from Carmichael Brothers, and our family sailed up the east coast of Australia and into Southeast Asia. Our hometown, Wakefield, is the last safe place to swim before you enter croc territory, so Summer and I and our younger brother, Ben, spent a lot of time on that cruise playing inside our yacht.

I loved everything about Bathsheba. She was a custom-built sloop, her sleek aluminum hull fitted out with the best timbers—teak decks, oak cabinetry—but what I loved most of all was the ingenious double mirror in the bathroom. The builder had set two mirrors into a corner at right angles, with such care that I could scarcely discern the line of intersection. When I looked squarely at either one of these mirrors, I saw Summer, as usual. But when I stared between them, past that line, into the corner, I saw a nonreversed image. I saw my true self.