Originally Posted by Terrapin_Station

First, keys aren't higher or lower than other keys. If we modulate from C to G, say, did we modulate up a fifth or down a fourth? (Or up a twelfth or down an eleventh? etc.) Keys don't tell you what pitches will be involved with respect to C3 (130.81 Hz when A=440) versus C4 (261.63 Hz when A=440) etc.

Next, pitches can be higher or lower without being in a different key. The pitches can simply be shifted up or down an octave or two or three, or whatever.

With piano/keyboard sheet music, especially with reductions of pop tunes and the like, it's generally written so that it's centered on middle C (which is C4), with the melody mostly in the range of A3 to A5, say. That's because it's easier to write and read--it avoids a bunch of ledger lines and "8va" and "15ma" etc. notations.

At any rate, so the first thing you need to do is determine whether something is really in a different key, or whether it's simply shifted up or down an octave or two (which makes it still in the same key).

If it's really in a different key, that might often be because the artist tuned to something other than A=440. Often music will be written "in the key it's played in" rather than the key it sounds in relative to A=440. In other words, if a guitarist tunes down a half step, but they play an open A string when they're playing a song (and let's say that open string corresponds to the tonic key-wise), then the sheet music, the tab, etc. is going to be written in A rather than being written Ab (or G#), even though relative to 440 it sounds as Ab(/G#). That's because the tune will be much easier to play in A than in Ab. In fact, it might even be impossible to play in Ab, depending on just how things are voiced. For example, maybe there's a part with an open sixth string E with the top part of the chord being a four-note chord above the twelfth fret--that wouldn't be possible to play a half step lower (and not just because there is no lower note than the open sixth string).

If it's in a different key and that's because of a tuning difference, the easiest thing to do is to change your tuning.

(And it's worth noting as an aside that it's not unusual for guitarists to tune to something like A=430. That's really tuning just a quarter tone flat (relative to A=440). So it's halfway between Ab/G# and A. If someone tunes to an "in-between" pitch like that, then the only way you could match their pitches when playing along (bar something like a fretless guitar or great skills with bends) is by changing your tuning.)

If it's simply shifted an octave, that should be easy for you to do, but you need to know what notes you're playing and not just read tab. For example, you should know that if you fret the fourth fret on your fifth string (the A string), you're playing a C#, and likewise you should know that if you fret the sixth fret on your third string (the G string), you're likewise playing a C#, only an octave higher. It's important to be able to quickly pick out the same pitch (all the C#s for example) everywhere it occurs on your guitar. If you can do that, you can easily transpose octaves (with the only problem being if the octave in question isn't available to you because of the range of the guitar).

If it's in a different key where that's not a tuning difference--say that something is written in C but sounds (relative to A=440) in F#--then you need to be able to transpose intervals. In this case, from C to F#, you need to transpose everything an augmented fourth/diminished fifth (it's the same thing) up or down (in other words, up or down six half steps or six frets). Being able to transpose to different keys--remember the keys themselves are not higher or lower--is also an important skill.