Whenever you see that little ® symbol, it means that the accompanying word, phrase, or device has been registered as a trademark. In the United States, this is done by the U.S. Patent and Trademark Office. You apply to have your trademark registered, and if they give you the green light, the trademark is added to the Principal Register of trademarks.

A trademark doesn't have to be registered. If you use a trademark without registering it, you get common law trademark protection, which is decent but far from perfect. The advantages to changing your ™ to a ® are:

  1. The trademark gets protected nationwide. If you don't register, the trademark is only protected in the areas where you're using it, or where you might be expected to expand in the future.
  2. If someone is trying to import infringing products, you can directly ask United States Customs to seize those products at the port of entry.
  3. After five years, the trademark is deemed incontestable, which means that you can get a rapid injunction to halt infringing use, and you won't have to prove its validity if you want to sue someone for infringement.
  4. If you sue, you can go directly into federal court without having to separately establish your right to do so. Otherwise, you usually can't get into federal court unless the dispute falls under diversity jurisdiction (that is, the plaintiffs and defendants are in different states, and the damages claimed are over a specified amount).
  5. If you win, you can get your attorney's fees from your opponent, as well as a court order to destroy all the infringing products. If the infringement claim is strong enough, you can even get three times the amount of damages you've proven.

However, there are several types of trademarks that can't be registered:

  1. Trademarks that are deceptively or confusingly similar to other registered trademarks. This isn't necessarily linguistic: a Starcraft boat is unlikely to be confused with StarCraft the game. But if you tried to register "SpaceCraft" to refer to a real-time strategy game, that would be deceptive or confusing similarity.
  2. Trademarks that consist of "immoral, deceptive, or scandalous matter," or "disparage or falsely suggest a connection with persons..., institutions, beliefs or national symbols, or bring them into contempt or disrepute." This is based on a "contemporary attitudes" test. The Washington Redskins came close to losing their registration after a group of Native Americans claimed the trademark was disparaging, despite the fact that few people had publicly objected in the preceding decades. (The lower court's judgment against the Redskins was overturned on appeal.)
  3. Trademarks that consist of government insignia, including the U.S., state, local, and foreign governments' flags, coats of arms, and other devices.
  4. Trademarks that consist of the name, portrait, or signature of a living person who hasn't given written consent. (Dead presidents count if their widow is still alive.)
  5. Trademarks that only describe (or deceptively misdescribe) their products. This one is kinda hairy, because depending on how you read it, it could exclude names like National Semiconductor or United Airlines. In practice, the PTO usually tries to see if a trademark would distinguish a product from competitors' products. So "The Semiconductor Company" or "Airplane Line" would be bad.
  6. Trademarks that are primarily geographically descriptive (wines and spirits like Champagne were excepted after the European Union raised hell at a WTO conference a few years back).
  7. Trademarks that are primarily geographically misdescriptive. The test here is whether the geographical description would lead people to believe that the product came from that place. You could trademark "Hawaiian shirts" made in California, but you couldn't trademark "California raisins" made in Hawaii. (Note that Europe is more picky, and won't allow any geographically deceptive trademark, regardless of whether anyone cares about origin or not.)
  8. Trademarks that are "primarily merely a surname." This is so people can't register "Smith's Grocery Store" and then keep anyone named Smith from operating a grocery store under their own name. Consumer perception is the basis for this judgment: very rare surnames are less likely to be recognized as surnames by the public, and so they are more likely to be registrable.
  9. Trademarks that are "functional as a whole." This is used to keep the domain of patent and trademark law separated. If a trade dress or product configuration is a functional part of the product, you're supposed to patent it, not trademark it.

Trademarks that don't qualify for the Principal Register can still be placed on the Supplemental Register. The latter register exists because many foreign countries require trademarks to be registered in their country of origin before they can be registered overseas. The Supplemental Register fulfills this requirement, but does not give any other benefits over common law trademark protection.
