The risks and benefits of three types of bubbles.

Connoisseurs of bubbles have observed at least three types.

The first is the Basic Bubble. It consists of whoever was living in a household pre-coronavirus — whether it was one person, a large family or a couple who want to break up.

The Basic Bubble can become a tense place when its inhabitants can’t agree on boundaries. A teacher in Salt Lake City said she hoped her three roommates would stay put. But two kept seeing people outside the household. Ultimately she and her top-floor roommate split with the first floor. They now communicate over FaceTime.

The next type you could call the Incorporated Bubble: people in one apartment or house invite friends, professional collaborators or relatives living in other households to move in with them.

Some do it because it seems like the safest way to continue to see people. Others do it to ward off loneliness. Soy Nguyen of Los Angeles, who moved in with a friend about a month ago, said this is why she’s still in good spirits. “Honestly, if we weren’t quarantined together and kind of lifting each other up, I don’t know how I would hold up.”

One challenge is that once you’ve started inviting other people into your bubble, it can be difficult to stop. Michael Nesmith, a high school football coach who lives with his wife and daughter in Maryland, had already agreed to let a niece and a cousin move in. Then his daughter asked if a close friend whose family lived far away could be a final addition.

“That was challenging,” Mr. Nesmith said. “I feel for this young man.” But he also felt responsible for the four women. “To run the risk of jeopardizing them for someone outside family, I just wasn’t going to do it,” he said.

Then there’s the Conjoined Bubble. The inhabitants of two or more bubbles get exclusive, agreeing to see one another while maintaining separate dwellings. Some people have practical reasons for this: they might agree to share home-schooling or babysitting with neighbors, for example.