The TV is on in the background, and you’re replying to a quick email on your phone nearby. You don’t know it, but the devices are communicating. During a commercial, the TV emits an inaudible tone and your phone, which was listening for it, picks it up. Somewhere far away, a server makes a note: Both devices probably belong to you. This information about which devices belong to whom is immensely valuable to advertisers hoping to target ads specifically to you. In a simpler time, targeted marketing was easy. Most people had a computer at work and maybe another at home. If you sent an email about your new cat, ads for cat food started cropping up. If you searched for Thanksgiving recipes, Safeway coupons for turkeys appeared in your Facebook newsfeed.