There is a category of trademarks which the U.S. Trademarks Office does not like. It's pretty unique because it's not really found in a lot of other trademark laws of other countries.
Ornamental trademarks are what the Trademarks Office says are unregistrable because they don't perform the function of a trademark. For example, if you have an image or a slogan that you're putting on your t-shirts and you want to get that trademarked just for the sake of "it's a good phrase", or "it's a good image", but the purpose of that is not to identify a product or a service, that's what the Trademarks Office in the U.S. may reject by saying, "Yeah, well, it's just purely ornamental. All you want to do is get the monopoly over the phrase or over the image. Not from the perspective of it being the identifier of your product, but you want to own the name, the phrase, the image itself."
As I've been telling many times, the function of a trademark is not to give you the monopoly over the name, phrase, or logo. It's to give you the monopoly over the mental link between that name, phrase, or logo and your particular products and services.
If all you're trying to do is say, "Well, that's a great phrase, let's trademark it," you may have an issue with the U.S. Trademarks Office. If it's a borderline scenario, if it's a grey area, then your chances of getting your trademark through go up significantly if you're being represented by somebody who knows how to deal with those things because a few words in your description and especially the way you submit the specimen of use—all of that could be crucial in terms of whether the Trademarks Office says, "Yes, it's a registrable trademark," or they say, "No, it's an ornamental trademark."
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