As if playing in tune isn’t hard enough on a normal day, try doing it in a lowered or dropped tuning. When your strings are tuned to a lower pitch, the tension is also lower and the vibration is much more erratic, which makes the pitch harder for a tuner to read. On top of that, it’s more difficult to get the lower part of the neck to play in tune, since there is normally no intonation adjustment at the nut, and it’s easy to squeeze the string sharp when fretting. Then, there is the fact that the harder you hit the string, the sharper the pitch goes on attack, going flat on the decay. So what can you do to correct for lowered tension, and get the string to have more tuning stability?
First let’s talk about strings, scale length, and pitch. There is a reason that a bass has a longer scale and larger strings than a guitar. In a perfect world, whenever you lower the pitch, you would increase both string size and scale length. For example, if you have a 25.5" scale-length guitar, and you want to drop your low E to C#, you would increase the scale length to around 26" or 27" and step up the size of the string from .046 to around .060. Another trick is to try strings with a larger core-to-wrap ratio. The tension and stability of the string can be increased with a larger core, and combining that larger core with a smaller wind adds up to the same gauge. A simple thing you can do with a normal set of guitar strings is to swap out the plain G string with a wound G string of similar gauge. So long as you intonate correctly, all of a sudden your cowboy chords will be more in tune.
I have worked with many bands that play in dropped-D or dropped-C#, where the whole guitar is tuned a half-step down and the low E is dropped an extra full-step down to C#. Again, this is doable on regular-scale guitars if you use heavier strings or a heavier core-to-wrap ratio. When the low string gets below C# you have real problems playing chords in tune. I have checked in with a few bands that tune low and they use tricks like hitting the string briskly when tuning and tuning to the attack, not the decay. Another helpful tip is to purposely tune a few cents flat on the low E and maybe the G string if it is a plain/unwound string.
The first few frets can be especially problematic—that’s where everything plays sharp and there is no intonation adjustment to fix it. There are some tricks to help: tuning systems like Buzz Feiten, the Earvana Nut, and the newer Hosco S.O.S. nut shim. The idea of the Earvana and S.O.S. is that you are shortening the distance from the nut to the first fret to flatten the pitch of the string near the nut. You have to make up the difference as you go up the neck by intonating the bridge saddles. Of course there are compromises on other areas of the neck, but the payoff of getting those first-position chords in tune is usually worth it.
The good news is that strings are relatively cheap and changing them is easy, so if you want to experiment with lowered tunings, you can do it without fear. Just be sure the nut slot is cut wide enough for whatever size string you are trying and check your intonation.