The messy thing about MBTI (aka That One Four-Letter Typology System That's Real Popular) is that there seem to be two main schools of how the letters are calculated:
The most common one I see basically picks one of two letters based upon your personality traits:
[I]ntroverted or [E]xtroverted?
I[N]tuitive or [S]ensing?
[T]hinking or [F]eeling?
[J]udging or [P]erceiving?
The other, however, thinks in terms of cognitive functions, which are a bunch of ways people (a) take in and (b) respond to information. Your four letters aren't determined by picking between one letter or the other, but which set of cognitive functions you use the most.[1]
For what it's worth, I find a lot of personality typing systems incredibly arbitrary, and this one is no exception. I do prefer the cognitive functions approach, though - even though it's IMO still drawing a bunch of boxes around the ways people work, it's at least more interesting to think about than trying to cram myself into a bunch of context-dependent personality dichotomies.
...the issue is, because the "context-dependent personality dichotomies" version is the most widespread one, that means that the main reason I'm sitting myself in these little boxes to begin with - sharing the results and comparing notes for funsies - is kind of moot. We're saying the same letters but they mean different things to each of us! (And let's not get into the stereotypes...)
[1] This is supposedly the Original and Authentic way of doing it before it got bastardized into the letter-dichotomy one by the Internet, but I haven't really cared enough to investigate this claim. As far as I'm concerned, they both describe different parts of a person - the main issue is trying to communicate which parts you're trying to refer to when people use the letters two different ways!
(fallen london stamps by