The Supreme Court ruled last week in the case of Fane Lozman vs the City of Riviera Beach, Florida. They decided that Lozman’s 60-foot, two-story, motorless, rudderless floating home was not a boat or a vessel, and hence should not have been seized under maritime law and destroyed by the city.

With the line between house and boat a little bit clearer, reader Steve asked us to clarify something else: “What defines a boat, versus a ship?”

One of the quickest ways to reveal yourself as a landlubber is to refer to a ship as a boat, but there’s no absolute distinction between the two, and even experienced mariners rely on local custom and usage to differentiate them.

Back in the Age of Sail, a ship was pretty well defined as a vessel with three or more square rigged masts. As different methods of power generation replaced wind and sail, the ships of old became more specifically known as “sailing ships,” and the usage of ship broadened to cover a wide, ill-defined variety of vessels.

One thing that sets a ship apart from a boat is size. According the U.S. Naval Institute, a boat, generally speaking, is small enough to be carried aboard a larger vessel, and a vessel large enough to carry a smaller one is a ship. Or, as Steve says his Navy Lieutenant father put it to him, “You can put a boat on a ship, but you can’t put a ship on a boat.”

Now, this Naval convention is a good rule of thumb most of the time, but there are a few exceptions, among both naval and civilian vessels. Some yachts, ferries, tug boats, fishing boats, police boats, etc. can carry small lifeboats or dinghies, but they usually don’t graduate to ship status because of that. On the other hand, a large container ship or the USS Cole can be carried aboard an even bigger ship without getting demoted to a boat.

The U.S. Navy seems to want to have it both ways with their submarines. One component of each vessel’s official name is USS—that is, United States Ship—but seamen, the Naval Institute says, usually refer to submarines in general as boats, regardless of size.

Another factor the Naval Institute considers is the vessel’s crew, command, and use. If it has a permanent crew with a commanding officer, it’s usually a ship. If it’s only crewed when actually in use and has no official CO, then you’re probably dealing with a boat. Ships are also usually intended and designed for deep-water use and are able to operate independently for long periods of time. Boats, meanwhile, lack the fuel and cargo capacity for extended, unassisted operation.

Again, though, there are some exceptions in actual usage. Most commercial fishing vessels, for example, are large and can go out alone on the open ocean for weeks at a time. They’re almost always called boats, though, and rarely “fishing ships.”

Without any hard and fast rules about boats and ships, we humbly suggest another loose guideline that will ingratiate you to the captain of any sort of vessel: Call it whatever the skipper wants you to call it.