Social media was not my friend that day. I stared obsessively at his photo on Facebook. It was one from a couple of years ago, before I’d known him. He was on holiday, sitting at a bar in Jamaica, drinking a cold beer. He was smiling and raising a glass in celebration; I’d always hated that photo, because I’d known he was smiling at Emma. Just as a stopped clock is right twice a day, a marriage is never completely unhappy. There will be days—or moments, even—when everything’s fine and you remember why you’re with that person. That photo seemed to have caught one of those moments in their marriage.

He and I had never been friends on Facebook. I rarely used it; it just seemed to magnify the faults in my life.

“Emma uses my phone sometimes,” he said when I asked him about it. “I don’t want to have to worry about whether she’s going to see messages we’ve sent each other. It’s not fair to her. And I don’t want to see what you’re up to with your husband. I don’t want to see photos of you both and see what a great time you’re having.”

I didn’t know exactly which recent photos of us would show that, but I knew what he meant. “What about Twitter?”

“I hardly use it unless it’s for work. I don’t use social media that much, really. Emma does, much more than I do. She loves Instagram.”

I wished he hadn’t told me that. As soon as he went into a meeting I turned my screen so that Sarah couldn’t see it and signed up to Instagram, using an alias. There was Emma, with gleaming blond hair and a huge smile. Her eyes were blue, her skin tanned, and she looked so confident, so feisty, that immediately I knew why Harry was with her. I felt sick with jealousy. I closed the screen and carried on with my work, but all I could think about was the way she looked at the camera straight on, her chin tilted upward, her gaze direct. She wasn’t someone I wanted to get on the wrong side of.