Would someone be able to identify this chord for me?
- F# - bass
I'm sure it's obvious but I can't seem to figure it out.
It's a song in the key of C major. A C major chord precedes it and an F major chord follows it
It really depends on context. It could be an Am6, but this is an unlikely inversion, so probably not. It could be an F#m7b5, especially if followed by some form of B7, where it would serve as a II in E, probably E minor. For other functions of half-diminished chords, see Wikipedia: Half-diminished Seventh Chord. It could also be a rootless D9 voicing with the third in the root, especially if followed by some sort of G.
If you can give us more context, we can better figure out how the chord is functioning and give you a more accurate answer.
Edit for updated question: This is F#m7b5 serving in a diminished function as a chromatic passing chord preceding the F. It could also be serving in a secondary dominant function, as a tritone substitution for the V of IV, if it can be considered an F#7b5#9 in a voicing without the third (think C/F# to better visualize its dominant function). It is described in more detail in the previously mentioned wikipedia article as a Sharpened subdominant with minor seventh, and appears in the Super Mario Bros. theme and The Entertainer (but not followed by the IV), although that section doesn't specifically address its function when preceding the IV.
In any event, I am now curious as to what song this is.
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It's an F#m7-5 (F# minor 7, flat 5), aka an F# half-diminished chord. It's the diatonic seventh chord built on the seventh degree of the G-major scale. In root position, it would be voiced F#-A-C-E.
It could also be a D7/9, without the D. In root position, it would be D-F#-A-C-E
Thanks for adding the context. Makes it much clearer it's a half-diminished seventh chord or, even more specifically in this context, a sharpened subdominant diminished triad with minor seventh.