Legal falls in Class 45, legal & security services, and depending on what exactly you sell it can also fall in Class 35 or Class 42 or Class 36. Everything below is read from the USPTO's own list of accepted descriptions, not from an opinion about where it belongs.
Searching the USPTO Trademark ID Manual for legal returns 172 accepted descriptions, and most of them sit in Class 45. The manual is the list of identifications the office has already approved, so wording taken from it is pre-approved and settles the class at the same time.
The class follows from the identification you write, so "what class am I in" is really "what exactly do you sell". How the class system works covers the rest.
Legal, security, or personal and social services.
156 accepted descriptions in the manual mention legal. For example:
You run a store, sell advertising or marketing, or provide business and office services.
8 accepted descriptions in the manual mention legal. For example:
Software as a service (SaaS), app development, IT, design, or science and engineering services.
3 accepted descriptions in the manual mention legal. For example:
Banking, insurance, investing, real estate, or other money services.
2 accepted descriptions in the manual mention legal. For example:
Legal appears in 4 classes because what you make, what it is made of and the service of selling it are separate classes. Pick the descriptions above that match what you actually do.
Legal is a service, so it sits in Classes 35 to 45. Software that delivers a service is often Class 9 or Class 42 as well.
Legal, security and personal services: legal services are here. Commercial services are not, so retail store services and the rest of business support are Class 35.
The class decides what a registration covers; it does not decide whether the name is available. Relatedness is judged on the identifications, so a conflict can sit in a class you never looked at: the same name in a different industry.
Check whether your name is clear for legal Searches the live USPTO register, including sound-alike and meaning-alike names. First search $2, then $5.Legal sits in Class 45, legal & security services. That is where the USPTO's own ID Manual puts it: 156 accepted descriptions mentioning legal fall in Class 45. Depending on exactly what you sell it can also fall in Class 35 or Class 42 or Class 36.
You file in every class you actually use the mark in, and the USPTO charges its fee per class. Selling a product and also running an online store for it is two classes, not one. Filing in classes you do not use is worse than filing in too few: an application has to be supported by real use or a genuine intent to use.
No. The class decides what a registration covers. Availability turns on whether someone already holds a similar mark for related goods, and relatedness is judged on what the identifications say rather than on the class number, so marks in different classes can still conflict.
Not what you sell? Search the class finder for your own wording, or browse all 45 classes.