Here’s an easy technique for the very common situation in which you are playing a song you don’t know the chords to. Assuming it is a fairly simple song with only I, IV and V chords, this works very well.
Are you already familiar with what I, IV and V chords are? If not, then here is a quick explanation.
Major chords are named just like the notes that make up all musical scales in western music: G, G#, A, A#, B, C, C#, D, D#, E, F, F# and back to G again. Many songs use only a few chords and the most common are G, C, and D if you are playing in the key of G. If you are playing in C, these same chords would transpose to C, F and G, In D, they would transpose to D, G and A. To save time doing all this transposing from one key to another when it really isn’t necessary, a numbering system was invented.
Let the root chord (that is, the chord that is the same as what key you are in) be the I chord (G if you are in the key of G). Then the IV chord is C for the key of G and D is the V chord for the key of G. That’s as much as you need to know for now. Check out Earl Scruggs’ Banjo book for more info on it. It’s on page 31 in the 2nd ed. and is called the Nashville Number System in his book.
OK, back to our technique.
First, you have to find what key you are in. Aside from asking ‘what key are we in?’ in the middle of the song, you can find it yourself with a little practice. Start by making a G chord with the F position. Pinch this chord and see if it fits the very first chord of the song (most songs start on the root, but not always – a better choice would be the very last chord, but by then, you’ve sat out one whole verse and chorus). If G isn’t the root, then slide up a fret. It’s probably not G# unless everyone is playing sharp on purpose. Keep sliding up the neck – to A, B, then C and D. With some practice, you should be able to find the root chord somewhere between your first and second G chords in the F position.
Now, assuming you have found the root fairly quickly, then with a little more practice, you’ll be able to pick up the next chord change. I’ve found an excellent way to pick up not only chord changes, but which chord it has changed to, is to listen to old country music. Many old country songs use only I, IV and V in various orders. Soon, you can start to hear that each chord has its own ‘flavor’.
From the F position root (say G for the key of G) you can easily make a IV chord (C for the key of G) by laying your ring finger down flat on the 5th fret. A V chord (D for the key of G) is made with the basic D position or even with a bar chord on the 7th fret.
Try these finger positions and you’ll see how easily you can switch between them. Remembering them as simply I, IV and V, you then have the whole fret board in any key at you disposal with very little brain work.