The reason why this is called an E-flat alto saxophone is because you need to play a concert pitch E-flat in order to get a C on the instrument. A concert pitch E-flat will sound like this. So that's concert pitch E-flat, and that equals C on the alto saxophone. An intervallic way of looking at that is, if you were to play a C on a piano or a concert pitch instrument, it's going to be an A on the alto, which is down a minor 3rd, but technically it's up a major 6th. It's an inversion of itself.
The reason why this is called a B-flat tenor saxophone is because it takes a concert pitch B-flat to produce a C on the horn. Same fingering as what I did on the alto. A concert pitch B-flat sounds like this on the tenor, which is C, okay? If we were to take another intervalic example, a concert pitch G is going to be an A on this horn.
So if you played a G on a piano or a concert pitch instrument, you're going to have to play an A on this to get the same note.
Technically it's transposed up a 9th, but you can think of it as a 2nd up from concert pitch. Here's a little something in the key of G on the alto saxophone, which is concert pitch B-flat. Here's the same thing on the tenor saxophone, which is going to be in the key of C.
That's transposition for the saxophone.