aleph0: you might be asking (and it is a bit confusing) about AB and ScB degrees. In many, and all of the science concentrations, you can get either an AB degree, or a ScB degree, the latter having more courses, and usually some sort of research project and paper. Some AB degrees also have a requirement for a senior "capstone" project. (Brown uses the latin for degrees etc, so AB=BA at other colleges, ScB=BS. ) It is very hard, almost impossible to get dual ScB concentrations. You have ONE Brown undergrad degree, no mater what re undergrad concentrations completed. Now to confuse it a bit more, there are some students who do a 5th year. They usually are getting an additional degree, (Masters) in (one of ) the fields they concentrated in as an undergrad. (The CS dept has a very respected 5th year Masters for example.)
So you can have an AB or an ScB in a science concentration.