The Key of D — Bass

D major harmonises to D – Em – F#m – G – A – Bm – C#°; D minor to Dm – E° – F – Gm – Am – A# – C. Most of the country repertoire in this key lives on three of those chords — the tonic, the subdominant and the dominant — plus the relative minor when a song wants one darker turn.

The bass chair carries the root-and-fifth pulse everything else in the arrangement is timed against. Almost none of a chord is playable down here and almost none of it is wanted: the mix hears whichever note sits at the bottom as the chord, so the choice is which one, and when.

Learn the tonic-subdominant-dominant grammar in D until your hand goes there without asking, then add the relative minor, and most of the repertoire in this key is already playable.