Progressions in A — 5-String Banjo

A major harmonises to A – Bm – C#m – D – E – F#m – G#°; A minor to Am – B° – C – Dm – Em – F – G. Each chord links to its 5-String Banjo voicings, so a tune in A becomes a short list of shapes before the first repeat.

Then check the key against the short g: some keys let it ring free through every change, and some make it a note you have to work around. Knowing which is which is most of what "playing in a good key" means here.