What are the two characteristics that dollars must have to be accepted as money in our society?

Question

I think its the spider and the line along the side I'm not sure

but i think that the line along the side is one of them.I had that question before but i can't remember what were the exact answers.

Is this a question about a) the physical characteristics of a dollar as Kalin-jasm-sis suggests, or b) is this a question for a basic micro/macro economics course?

If a) then I don't know. If b), a dollar bill must (1) have the expectation that the dollar will be accepted as a form of payment anywhere else in the U.S., and (2) have the expectation that the dollar will retain all or most of its purchasing power in the near future.