Different courts use different tests, but all of them tend to focus on the same factors. The confusion analysis at the United States Patent & Trademark Office (“USPTO”) uses thirteen factors:

1. the similarity or dissimilarity of the marks in their entireties as to appearance, sound, connotation and commercial impression;
2. the similarity or dissimilarity and nature of the goods or services as described in an application or registration or in connection with which a prior mark is in use;
3. the similarity or dissimilarity of established, likely-to-continue trade channels;
4. the conditions under which and buyers to whom sales are made;
5. the fame of the prior mark;
6. the number and nature of similar marks in use on similar goods;
7. the nature and extent of any actual confusion;
8. the length of time during and conditions under which there has been concurrent use without evidence of actual confusion;
9. the variety of goods on which a mark is or is not used;
10. the market interface between applicant and the owner of a prior mark;
11. the extent to which applicant has a right to exclude others from use of its mark on its goods;
12. the extent of potential confusion; and
13. any other established fact probative of the effect of use.

In re E.I. DuPont DeNemours & Co., 476 F.2d 1357, 1361 (C.C.P.A. 1973).

The first and second factors are the most important, although the fame of the prior mark, evidence of actual confusion, very long-term co-existence, and consent agreements between the parties also carry considerable weight, if present.

The USPTO and its reviewing courts, the Trademark Trial and Appeal Board and the Federal Circuit, focus on the details of a trademark application or registration – the goods and services as described and the typical usual channels of trade and customers for those goods and services in general, rather than the actual goods or services or channels of trade used by either party. So, if the USPTO considers an application for a mark for “dresses,” it will conclusively assume it include all types of dresses, even if it is only used for wedding dresses. A state or federal court, by contrast, will focus on the actual market conditions in which marks are used by their owners. For more on the differences between the Trademark Trial and Appeal Board and federal and state courts, please read our guide "What Is Trademark Infringement?"