One of the most interesting things I’ve experienced while using the Apple Watch over the past two and a half months is the growing consciousness that I am now wearing several machines that are capable of talking to each other.

This was illustrated one morning when I was in the New York City subway, deep underground. There—far from any cellular signal or internet connection—I used my Apple Watch to wirelessly change the podcast that was playing on my iPhone, which was being wirelessly transmitted to a pair of Bluetooth earphones. I had my own invisible, personal network, off the grid.

This phenomenon—some are calling it the “personal cloud” or “personal mesh”—is one of the most important changes that will come out of the rise of wearable devices, such as the Apple Watch.

While early consumer smartwatch applications have focused on fitness tracking and messaging, the more powerful capabilities—for entertainment, medicine, and life in general—could come when millions of people are each carrying around several devices and sensors. Today, that includes earphones, Bluetooth hearing aids, wireless heart-rate monitors, and running-shoe sensors. Soon, it could include clothing with sensors built in to their fabric—already in the works, but not yet mainstream—and sensors implanted in the body.

These personal clouds are interesting on their own, but especially when they’re joined, or connected to larger environments such as cars, buildings, or cities, carrying context, personal preferences, and data.

“If the first wave of the mobile phone’s impact on the tech sector was driven by applications running on the phone, the second wave will be driven by the phone connecting to other devices, including other phones,” tech investor Fred Wilson—who made early bets on Twitter, Tumblr, and Etsy—wrote last year in a blog post about the personal cloud.

It’s easy to get carried away with hypothetical examples. In a world where everyone shares communal, self-driving cars, yours will always know your seat settings, music preferences, and Starbucks order.

But there are already practical uses for the early adopter. Starwood, one of the world’s largest hotel chains, is in the process of outfitting thousands of rooms with new locks. Now, instead of waiting in line to check in at the front desk, eligible visitors can do so quickly and privately from their smartphone app, and walk directly to their room. A Bluetooth signal from your personal cloud—today, a smartphone or Apple Watch—will unlock the door.