If you tune your banjo to a standard open G tuning, gDDBG, you learn the chord positions using those open strings and learn the "licks" using the same avaialable strings. That is OK as long as the tune you are singing or playing is written and perfromed in the key of G. If you want to play with a fiddler has a tune written in the key of A, you have 3 choices: 1. tune your banjo up 2 frets so you can use the same fingerings 2. learn new fingerings using closed positions 3. use a capo.
Using a capo at the 2nd fret effectively tunes your banjo to an open "A" chord so you can use the same fingerings as you did in open "G".
The whole tones of a G scale in western music are: G A B C D E F# G The banjo has frets for the spaces between the whole tones, so the fret arrangement in G tuning is: G G# A Bb B C C# D Eb F F# G. So if you apply the capo at the 1st fret the key is G# (or Ab) 2nd fret A 3rd fret Bb. 4th fret B 5th fret C 6th fret C#(or Bb) 7th fret D, 8th fret Eb(D#) 9th fret E 10th fret F 11th F# 12th fret G
__________________________ "It is better, of course, to know useless things than to know nothing." -Seneca
There are several ways to view capos and all of them make sense and confusion simultaneously.
First off, try understanding closed chord forms without the capo.
Play your G-C-G-D chords up there at the 9th fret G: 9789 C: 10-9-8-10 G: 9789 D: 7777
Important fact #1. If you slide the whole thing up one fret and strum the chords (without using the 5th string), you will now be playing Ab-Db-Ab-Eb. If you slide it up two to start with the little finger on the 11th fret instead of the 9th, you will be playing in A: A D A E
Important fact # 2. If you play the regular G-C-G-D starting at the 9th fret you will be playing in the key of G. If you put a capo on the 2nd fret and play those same chords at the same place you are still playing in G[/i/]. If you put the capo on the 4th fret and still play these chords, you will still be in G. And if you slide the chord pattern up two frets, you will be in A [i]regardless of where you put the capo.
I think that's the easiest way to understand capo use: not from looking at what changes but looking instead at what doesn't change. ====================
Here is the second way I explain capos. Let's move the entire G pattern down two frets. Now you are playing in F. Now move the entire pattern all the way down to D chords.
Notice that these are the same as the G shape-chords you started with. D: 4234 G: 5435 D: 4234 A: 2222
Now, slide the D chord down one fret to 3213. That is a Db or C#. Now slide it down one more fret. You must lift a finger to make your C chord (probably the first or second chord you ever learned): 2012
That's the first half of this exercise. If you wanted to try playing your G-C-G-D or D-G-D-A form (they are identical shape chords), you will end up playing C-F-C-G using the 'standard' chords most folks start with: C: 2012 F:3213 and G:0000
Now let's start from D again 4234 but this time, put the capo across the second fret. Now you will be playing in the key of D because the chord sound will be D but the chord you form on the banjo will be a C: 2012 and the A chord will be a G open: 0000.
When using a capo, if you play up-the-neck closed chords, nothing changes. But all the chords down close to the capo that now use open strings do change. ===========================================
To understand where to put the capo in order to use it, first form your base chord at the lowest possible point on the neck where you can fret all 4 strings.
Let's try the key of F. The lowest chord is 3213. If I want to put a capo on, all I can do is put one on the 1st fret and play an E chord instead. I can't go any higher with the capo because I've already started using open strings on my base chord.
Let's try the next higher actual F chord: 7567
Make that shape on the banjo and then look at where it might make sense to put a capo. If you put it on the 5th fret where it takes away the 3rd strings' fret, then you will be playing in C position: 2012 from the point of view of the capo.
If you put the capo on the 3rd fret, you can be playing out of D: 4234 (from the point of view of the capo (add three frets to each number if you aren't doing this on the banjo neck)).
Good luck. Capoing is one reason many folks use the numbering system for chords.