Wow, you can study one thing for your undergraduate and another thing for your masters? I'd love that here, but you need to have studied a "relevant subject" (i.e the same thing but at undergraduate level) to even be able to apply. So you definitely get more options on what to study in the US vs many other countries? (we have a similar "you have to study x at "high school" level to be able to study xyz at university)
Well, history was the connecting thread for all my degrees (I did it as an undergrad and then as a grad student), so that was in the same ballpark. And again, it varies -- if you’re applying to do a hard science/physics/mathematics/engineering degree at graduate level, you need to have studied it at undergrad level first (and the only people applying for those grad programs are the people who have studied science all the way down and are intending to progress in it). But with arts/humanities/social sciences/law, there is more leeway, yes. My friend @extasiswings went from a music performance undergrad degree to law school, so people can move fairly considerably between bachelor’s and master’s level. By the time it gets to PhD, it’s again more specialized in requirements, but yes, you have the option of ending up in master’s study somewhere quite different than your first degree (and a career that may not relate to either).