If there's one thing sure to detract from your performance when things otherwise are going well, it's not being in tune. But there is much to know in order to play consistently in tune. Assuming your instrument is good that is, has good intonation (the frets, nut and saddle are all in the right place and the action is set right) you need some good information. Playing in tune is certainly easier said than done, but here are some tips to help ensure your audiences aren't tempted to throw tomatoes at you:
1.Rough in the tuning the traditional way of matching the 5th fret of one string to the next string open, except for the 3rd string, which is held at the 4th fret to get the pitch of the open 2nd string.
2.Refine this by checking octaves, fifths and major thirds (see Theory if this is new to you). Start by checking the two outside strings, both E's and two octaves apart. Now, one of these has to be left alone, and the other one is the one that gets adjusted. I prefer leaving the 1st string as the reference pitch and moving the 6th if necessary. The two need to blend perfectly. By blend, I mean the two strings don't create waves when sounding together.
3.Now check the 6th and 2nd strings together. These two should blend very well.
4.Repeat with the 1st and 5th strings. The advantage of doing open strings together over the 5th fret, 5th fret, 5th fret, etc method is that both strings are in the same state, open, not just one of the strings. It also is good for helping you learn to recognize intervals other than unisons (two notes of the same pitch.)
5.Now check all the E's. Recheck the open 1st against the 2nd at the 5th fret, and adjust the 2nd string if necessary. Check the open 1st against the 3rd string at the 9th fret and adjust if necessary. Check the open 1st string against the 4th string at the 14th fret. Check the open 1st string against the 7th fret harmonic on the 5th string. Check the open 1st string against the 5th fret harmonic on the 6th string.
6.Now check major thirds with the chords, E, A and D. So, for the E chord, check the open 6th string against the G-sharp on the 3rd string, 1st fret. Also the 6th string E against the 3rd string 13th fret.
7.Now repeat this procedure with the open 5th string A against the C-sharps on the 2nd string at the 2nd and 14th frets.
8.Last, check the open 4th string D against the F-sharps on the 1st string at the 2nd and 14th frets.
Check tuning for each piece you play. If you're in the key of G, check G, C and D chords. It will be different for the key of E. And as you play any piece, check any suspect intervals that come up repeatedly when you get to a certain place in the piece. Before performing in public, check those spots first and make the little adjustments.
You may also want to look a little deeper into the puzzle of why tuning the guitar is inherently difficult. For the answer to that and more tuning advice, see luthier Bruce Walker's "Zen and the Art of Tuning the Guitar."