Depends on the make and year of boat you will be installing this into. The bases are different among manufactures. I know there are 2 of not 3 different bases and sometimes they change with the manufacture based on the model year. Springfield and Swivel-Eze are the 2 most popular manufactures. From there you'll need to determine which base you have to get the correct fit.
Lots of choices on seats....some like the version you can sit on and others like the version you lean against.
Personally, I like the kind you can sit on.