How many letters of a trademark could be considered infringing on a trademark.

You, in good faith, asked a hypothetical question. As has been made abundantly -- and overly -- clear, there is no answer to your hypothetical.

But you also asked a specific question: " ... can this case be used as a precedence in first case?" The answer is an unequivocal "no." Unless the parties to Case 2 are the same as those in Case 1 and the new dispute is based on the same facts and circumstances of the former case, then the outcome of Case 1 is not a "precedent" that binds the court that hears Case 2. Lawyers argue their case based on the facts and the applicable law of that particular case -- Bad Guy did x, y, and z and so the controlling law says that Good Guy is entitled to ... something. It is sometimes "persuasive" to bring to the court's attention a different case with similar facts and law and to argue BY ANALOGY that the outcome of the present case should be similar to the outcome of the former case. But that argument can only be used to seek to persuade the court (sometimes quite strongly, sometimes not so much), not to control the outcome of the present case.

There's no answer to this type of abstract question and no bright line rule about identical letters equating to infringement, because consumer confusion over two competing trademarks can arise because of the marks' look, sound, meaning, or any combination of those factors.

So it depends. Some marks are confusingly similar having only a few letters in common, especially the first letters (or word, or words) because the USPTO considers those more important than subsequent letter (or a subsequent word or words), because consumers think they're most important in indentifying a mark and the source of the goods or services.

There are 400 lawyers in the USPTO office who act as "examiners" in reviewing trademark applications and who decide whether an application has a mark that's confusing similar to existing mark. These lawyers have guidelines that they use, but they also have a lot of discretion, like judges do in court cases. They're known to be quite susceptible to precedents set by other examiners, and they don't want to outside the norm of their office and don't want to be"reversed" by a senior/managing lawyer, eso if there's a case of a denial like your hypothetical, then yes, you'd absolutely use it if it's in your favor.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

Notice: The below is for educational and informational purposes only, is not a reflection on or a representation of any views or opinions held by my employer, and should not be construed as legal advice. Nothing herein is meant to create an attorney-client relationship.

As a former Trademark Examining Attorney at the USPTO, I second Ms. Koslyn's comments regarding individual discretion in refusing or approving a given trademark application.

However, your question is broader than whether or not the USPTO will allow a trademark application to be registered when it contains some of the same letters as another mark. Nevertheless, even outside the confines of the USPTO, your question is simply too theoretical and abstract to be answered in this forum. Each case of potential infringement needs to be analyzed on its own merits and facts. It is impossible to answer such a question without knowing the details of both parties' uses of their marks, the goods/services at issue, consumers, channels of trade, geographic use, whether confusion in the marketplace already exists, who has priority, etc. If your question relates to an actual concern that someone may be infringing upon your rights or that you might be encroaching upon another's rights, I suggest you retain a knowledgeable trademark attorney to review your case. I caution you against providing any actual facts on this public forum.