Often when one is asked "what is the key signature?" they will respond with the particular sharps or flats that are present in the signature.

Yes, but…

While they may be correct in recognizing the sharps or flats present, the relevant fact is what is signified, or signaled by a particular key signature.

Properly identifying the key signature predicts the future, and it's easy to do.

Imagine that you are sight reading a piece. You've never seen this piece and you will shortly need to play it to accompany a violinist in her lesson in five minutes—or, even worse, you're asked to sight read it for the first time in a lesson! In front of you is a page of densely-written music. At first it just looks like black specks on the page. Knowing the key of a piece is a giant leap in making sense of the notes on the page. 

Knowing the key in which you will be playing predicts the future in many ways, including:

1. Tonic pitch (home note) - Everything follows from identifying the tonic pitch from a key signature. Many errors in your sight reading can be forgiven if you land on tonic at the right time: the ends of individual phrases, and sections—and especially the end of the piece! Think of how excited the judges are when a gymnast who is having a rough performance "sticks the landing!"
2. Mode - The key signature itself suggests two possible keys, but a quick scan of the music can reveal whether the music is in a major or minor key. Look for accidentals—in particular, the leading tone of the relative minor key—and most certainly look at the notes and chords at the end of the piece. Do they belong to the major or minor key possibility?
3. Scales - Scales are perhaps the first thing that comes to mind when thinking about key signature. We're all familiar with the image of the musician endlessly practicing scales. The major scale will be prevalent in major keys, while the various forms of minor scales can be anticipated in minor keys.
4. Chords - Western tonal music is constructed around the tonic (I/i) and dominant (V) chords. Add to these the sub-dominant (IV/iv) chord and you have what are called the Primary Chords. If I know I'm in the key of C major, then I will expect to encounter many C, F and G major chords. Other chords to expect are the ii, iii, vi, and viiº chords (these latter being minor chords, save for the viiº, which is diminished).
   
   Furthermore, I can expect to see D and A major as well, as these are the tonic (I/i) and dominant (V) chords of G major, which itself is the dominant chord of our original key. Pieces in the common practice period tend to move from the original key to the key area of the dominant. This creates harmonic movement within sections and serves to differentiate one section from another.
   
   Knowing all of the chords in any key (these are called the diatonic chords) will prepare you to play a variety of music. This can be done by learning common chord progressions in all keys.

So how do I predict the future?

Every key signature suggests two possible keys, a major key and its relative minor key. Identifying these key possibilities from a key signature is very easy.

Remember this for key signatures with any number of sharps:

For key signatures of sharps, a step (2nd) up from the last sharp is tonic of the major key, while a step (2nd) down is tonic of the relative minor key.

Notice that by step it is assumed that you are to go to the next or previous letter in the music alphabet. 

In this example, you have one sharp in the key signature—F-sharp.

A step up from F-sharp is G and a step down is E, thus a key signature of one sharp is either G major or E minor.

There are two ways to approach key signatures with flats.

This first way works for key signatures with any number of flats:

The tonic pitch of the relative minor key is always up two steps (3rd) from the last flat, while tonic of the major key is down three steps (4th) from the last flat.

In this example there is one flat, B-flat. Up two steps (the interval of a 3rd) from B-flat is D (relative minor tonic), while down three steps (the interval of a 4th) is F (major key tonic pitch).

Another way, which only works for flat key signatures with two or more flats (but requires less counting) is as follows:

The second to last flat is tonic of the major key, while tonic of the relative minor key is two steps (3rd) down from the second to last flat.

That is (summing up these two methods),

Second to last flat and down third; or very last flat and up a third. 

Using this method requires less counting, as the first step identifies the major tonic note directly (second to last flat!), but it only works with flat key signatures of two or more flats.

In conclusion, learning to quickly recognize key signatures is essential, as knowing what key you are in—whether sight reading or improvising with a band—is essential.

Use these tricks at first, but aim to commit your key signatures to memory soon!

Now go to any music you have and skim through the pages, identifying the major and relative minor key possibilities suggested by each key signature you find. Concluding exactly what key the music is in requires a few more steps, but this is an invaluable step on your journey!

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