What is this chord>
 

I often use (what I call) a passing chord from A to wherever.

I go from A (X02220) to ? (X12020). What is this chord?

Steve

B flat dim7th

Perfect! Thanks.

Steve

I really like this jamplay tool for figuring out things like that. https://www.jamplay.com/tools/guitar-chord-finder/

beats me, im no musician, this could be number of different chords depending on the inversion- this has the note structure of a G diminished

As mentioned, it's where it's going that could determine the best name for the chord.
It's certainly a dim7 of some kind, and could be named after any of its notes (with some enharmonic flexibility too).

As jerry says,if it's heading for a B-root chord (B major, Bm, B7), then the theoretically correct name is A#dim7, because A#dim7 is the vii chord from B (harmonic) minor. So it naturally resolves to Bm,and can be borrowed to resolve to B major. The notes (stacked in 3rds on the root) are A# C# E G, and A#-G is the defining "diminished 7th" interval (half-step less than a minor 7th).

Given that you're starting from A, that's the most likely identity of the chord.

But as it's a symmetrical chord, it can go three other ways, acting as vii to three other chords (again, major or minor in each case).

As C#dim7 (C#-E-G-Bb), it will go to Dm, D or D7 (that could also be a likely scenario starting from A);
As Edim7 (E-G-Bb-Db), it will go to Fm, F or F7;
As Gdim7 (G-Bb-Db-Fb), it will go to Abm, Ab or Ab7 (and if you call Abm G#m, then the dim7 is Fxdim7: Fx-A#-C#-E, strictly speaking).

Those all apply whichever note is in the bass. E.g, even if Bb is in the bass, if the chord goes to Dm it's C#dim7 (and the bass would then be Bb, not A#).
Still, this is only about theoretical correctness (proper enharmonic spelling for a vii chord)! In practice, it's normally sensible to call a dim7 after its bass note, not its functional root note.

THIS is a great online tool

Either Bbº7 or A#º7. It depends on your starting key and where your going harmonically after the º7th chord.

I understand the "#" as meaning "sharp" and "b" as meaning "flat", but what the heck is the little zero thing mean and where do you even find it on a keyboard to type in?

Steve

Little zero means, "diminished."

And on a Mac, it is option-zero. No idea how to do it on those other machines. ;)

You need a superscript "o". I generally just type an o, and I don't know any way of getting the superscript version on a normal PC keyboard - at least not when typing in forums.
In a text program you can use the keypad to get the ASCII code: hold Alt and hit 167 on the number keypad. And then copy it in: º
You can get the half-dim symbol that way too: Alt + 155: ø

And this site - https://www.reddit.com/r/musictheory/new/ - has all these useful symbols in the sidebar for copying and pasting: ♯ ♮ ♭ ø ° Δ ♩ ♪ ♫ ♬