Oh, my head is spinning--Why are we talking about this using an F#dim--it makes it very hard to figure out examples!

First, note that F#-7b5 is a different chord than the Fdim(sorry, but everytime I try to hit the degree sign, it starts AOL Instant Messenger) One has an E, the other is a half step lower--

Second--the use of enharmonic note names is determined by the key that you are in(names of chords, too)--When you are in Ab for instance, the note is called Eb, but in the key of E, it is D#-in the naming of notes, it makes a difference whether the note is in the scale or not--you wouldn't call an D# an Eb in the key of E--

A couple things to keep in mind--at least as far as Leon White is concerned, his concern is that you understand what is expected when certain chord symbols appear in music, and what they indicate--on occasion, the symbols are not precisely right, for one reason or another--For instance, many times you will see a C-Edim-Dminor progression, when what is really wanted is C-C#dim-D minor, a small point, perhaps, but important to say, the person playing the bass line, whose part moves up from C by half steps--

Again, the rule is that the naming of notes and chords is based on the key signature, and it's related keys, so our F#-7b5 may be called an Am6 in another key, and those damned full diminished chords can each have, well, at least eight names each(A guess, I haven't time to figure it out right now)--

The critical thing to understand is that the chords changes really highlight motion in the melody, and the same chord, in a different key, provides a different kind of motion. A move from, say, G to F#dim is a completely different effect than from F to F#dim, and F# to F#dim is different again--