The blanket “one-size-fits-all” rule
If customers are likely to be confused about the source of your products or services and think they are from another company, that’s likely trademark infringement.
Infringement is always fact-dependent and must be analyzed on a case-by-case basis. That means it depends on exactly what the original thing is, and what the alleged infringement is. So the “blanket-one-size” rule has a corollary rule: It depends on the facts.
There are two questions here: Are your and your competitor’s trademarks confusingly similar, and are your goods/services confusingly similar to their goods/services? The similarity need not be EXACT. Just as copyright infringement doesn’t require identical copying, trademark infringement only requires that the mark be confusingly similar.
Not all exact trademark matches are a problem.
Consider DELTA. It’s the name of an airline. It’s also the name of a company that sells bathroom and kitchen faucets and sinks. No customer is likely to mix up those companies when shopping for airline tickets or faucets/sinks, so that’s an example of trademarks that can peacefully co-exist without causing consumer confusion.
The legal test courts use in determining “likelihood of confusion” is a deliberately vague standard
Legislators didn’t want to choose some arbitrary number like 10% and say, “as long as you change at least 10% of the original, that’s different enough.” There is no magic number that works to avoid confusion. If, for example, a famous brand’s clothing is recognizable because of its rivets and stitching, even if you put your own brand’s label on that clothing, too, then no matter how different your brand is from theirs, that’s considered confusing. The more famous the brand name and goods, the more likely that brand is to sue you not only for infringement but also for “tarnishing” and “diluting” their brand. In fact, famous brands are entitled to broader rights than non-famous brands to protect their investment in their brand, which is typically millions of advertising dollars.
You need to “clear” your selected mark before using it.
The last thing you want is to launch your new company and receive a quick “cease and desist” letter from a lawyer for a competitor who beat you to market and chose a trademark that is confusingly similar to yours.
In practice, what does confusing similarity mean? It means that adding an “s” to the end of someone’s trademark or adding “the” at the beginning won’t legally distinguish your mark from theirs. What else almost certainly won’t help? Some or different punctuation. Font style or size. Spacing. Changing upper or lower case lettering.
What about confusingly similar goods/services? All trademarks are in one or more of 45 classes. Like pet food (class 031), pet toys (class 028), and pet clothes (class 018). Those are all in different TM classes, but they could be made by different companies, and consumers are very likely to be confused if the trademarks are also the same or confusingly similar.
The way to avoid being sued for infringement is to create your own distinctive “fanciful” and “arbitrary” trademark, like a piece of fruit as the source of computers and phones and music, not a mark that simply says what goods/services you provide, like “Jane’s Shampoo.”
Hire a professional. USPTO fees are non-refundable, and mistakes cost time and money, as each question (known as an “Office Action”) from the USPTO Examining Attorney can take several months to respond to. Note also that about 1/2 of all trademark applications are rejected, but your odds of having your application accepted increase significantly when you hire a trademark lawyer.