Chair falls in Class 20, furniture, and depending on what exactly you sell it can also fall in Class 10 or Class 12 or Class 43. 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 chair returns 107 accepted descriptions, and most of them sit in Class 20. 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.
Furniture, mattresses, mirrors, or picture frames.
62 accepted descriptions in the manual mention chair. For example:
Medical, surgical, dental, or veterinary instruments and health apparatus.
13 accepted descriptions in the manual mention chair. For example:
Vehicles or parts for traveling by land, air, or water.
8 accepted descriptions in the manual mention chair. For example:
You serve food or drinks or provide lodging — a restaurant, cafe, bar, or hotel.
5 accepted descriptions in the manual mention chair. For example:
Chair 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.
Making chair is Class 20; a shop or online store that sells it is retail store services (Class 35), and a business that does both files in both.
Furniture, mirrors and frames. This class runs on what the thing is rather than what it is made of, which is why metal furniture is here while metal pipes are Class 6.
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 chair Searches the live USPTO register, including sound-alike and meaning-alike names. First search $2, then $5.Chair sits in Class 20, furniture. That is where the USPTO's own ID Manual puts it: 62 accepted descriptions mentioning chair fall in Class 20. Depending on exactly what you sell it can also fall in Class 10 or Class 12 or Class 43.
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.