Warning! Non-standard music teaching approach coming up! There are OTHER MORE COMMONLY ACCEPTED WAYS to do this...

...this is simply a way that I use to help people who ask: "How can I play along with the CD if I don't know what key it's in?"

OK, assuming that you are at the beginner or moderate stage of guitar (or banjo or mandolin etc.) prowess, here goes:

Get a music book and write down the 1, 3, and 5 notes of every MAJOR chord. (hopefully the song you're trying to play ISN'T in a complicated looking sharp or flat key...but we can deal with that)

You'll have:

Chord of C. "C,E,G" or chord of D. "D,F#,A or chord of G. "G,B,D" etc. til you've done 'em all.

Put on the CD. Now as you listen to it, play each note on the high (E) string of your guitar, fret by fret. (don't forget the open "E" note) Do each note about a second apart.(you need to memorize the names of the notes from open to the 11th fret)

If you've got an average (but untrained) ear, some notes are gonna make more sense than others. Chances are when you hit the note that REALLY IS the right key it'll simply sound "like it fits"

OK, you've played the CD, taken your time to hit each note on the first string, and the one that really stands out is....say... "F".

Go to your chart. Play the chord that starts with F. Chances are that's it. Key of F. See if the first chord of the song IS F. Fine you've nailed it.

But....supposing it DOESN'T sound like F is the key? No problem, look for another triad (that's what those groupings of three notes are in the chart you've made) with the F note in it. How about Bb? Try that as the first chord. (you may even have to capo up to the third fret and play a "G" chord to get the Bb sound, but that's okay.)

That sound right? You got it then. Song's (probably) in Bb.

But maybe not..... Still doesn't sound EXACTLY right to your ear? No problem. We've still got a third option. What other triad has an "F" in it? Oi Vay! it's the Db triad. Oh well, capo on the first fret and play a C chord. Maybe that's the one you're lookin' for.

If the song is in a major key and follows MOST of the rules, your song is gonna be in either the key of F, Bb, or Db.

With a bit of practice you can do this in about 20 seconds, which beats taking your CDs to a teacher to just TELL you what key the songs are in (spend the money on Delmore Brothers albums!)

Now this ain't foolproof, and maybe someone can mention the triads for MINOR keys, but it DOES seem to have helped a lot of the beginner and intermediate folks I've worked with.

Rick