What key is I Got You (I Feel Good) by James Brown in?

[Funky President]

I'm having trouble figuring out why people say it's in D Major.

I understand that since it starts on D it's said to be in D, but D Major has an F sharp and a C sharp.

The F sharp is only used for the arpeggio parts, like the one that ends the song. All of the rest of the Fs are played as naturals.

All the Cs that are played n the bridge section (When I hold you in my arms...) are also played naturally.

While I understand that key and key signature aren't necessarily the same thing, I still don''t quite get it.

[saxmachine777]

It's in D Major. The F naturals throughout are blue notes when they're on the I chord (D7), and they're in the chord for the IV chord (G7). All of the D chords used in the tune are D7, which is the shorthand of saying D Major, minor 7th. That lowered 7th gives you C natural. The confusing part is that though the tune is in D Major, all of the D portions utilize that D7 chord, which sets the tune modally in D mixolydian (DEF#GABC). It has a major third, so it's a major key, even though a flatted third is often used for blues connotation, and you'll likely never see a C#.

[JimK]

Originally Posted by Funky President

While I understand that key and key signature aren't necessarily the same thing, I still don''t quite get it.

...this happens a lot in R&B/Rock/ tunes.
Don't get too hung up on it.