https://banjobenclark.com/lessons/soldiers-joy-mandolin
A classic fiddle tune in the key of D! This one is simple enough for all to learn, but with some cool tricks thrown in the second A and B parts…I love those chromatic runs! Have fun with this one!
This question isn’t specific to this lesson, but this lesson fits the question…
I’ve noticed with every song I’ve learned, the chord structure for a song is nearly never the same from source to source. I’m guessing no one arrangement is right or wrong, but in a jam situation, what do you do?
Using this lesson as an example, the chord structure of the B part is
DD GG DD AA
DD GG DA DD
Other sources I’ve seen have A chords in place of the G.
DD AA DD AA
DD AA DA DD
So where one person might have learned the song with a G chord, another person might have learned it as an A. Do you just need to adapt and switch up your learned chord structure on the fly to match the jam you are in? I imagine anytime you are playing with others, you have to have some flexibility. Can you learn it the way this lesson is arranged, and just chop a G chord in the 2nd measure of the B part even if the guitarist learned it with a different structure and is strumming an A?
I would probably play whatever blends best with the rest of the group. If you’re on mandolin, there’s nothing wrong with just doing a “dry chop” during those chords (a chop with just muted strings and no real tonal value). If you’re on guitar, you could probably play a partial chord if you felt like it. If on banjo, it matters less since we hardly ever follow good musical practices anyway 
Sometimes I’ll just roll over the root chord if there’s a quick chord change, etc… I feel like it’s fine to just drone there.
Specifically with Soldier’s joy, technically that second chord would be an A7, which is an A chord with a G note added in. This would probably explain the confusion between A and G in different chord structures.
Are you saying that an A7 is close enough to an A or a G, so either would probably blend in?
What “technically” makes it an A7?
Also, if it is technically an A7, why would @BanjoBen teach it as a G. I realize that question might sound negative, but it’s not meant to be. I’m trying to figure out how much of this stuff is “hard rules” vs preference vs creativity.
Thanks for your input.
The reason an A7 is an A7 is because it is an A chord with the flat 7 of the A major scale added in. I would say that I personally like the mentioned chord in Soldier’s joy as an A7, but that’s just 'cause Earl played it that way on banjo… If doing it with a band, probably the best way would be to have the entire rhythm section just play A or G. It probably wouldn’t matter quite as much for mandolin since the chop can just be muted if it sounds bad, fiddle since it’s either chopping or more individual notes vs. strumming, or bass since it’s mainly playing root.
What I meant to ask is why you were saying that specific measure in Soldiers Joy is “technically” an A7, not what makes an A7 an A7.
I meant that because the song is sort of resolving to a 5 chord there, but the melody makes your ear want to hear the 4th tone, that an A7 would be very well placed there, and it is sometimes played that way by lead instruments as well, for example, Earl Scruggs’ banjo arrangement. It’s not a hard and fast rule; different groups play it differently. All I’m saying is that based on the feel of the song, that I would probably want to play an A7 on guitar there. If some people in your jam play a G and others play an A, an A7 can be a nice compromise.