The concept of musical “keys” can be a complicated and confusing topic, so let’s give it a go…
For the most part, specifying a musical “key” is simply a way of making clear:
(i) The pitch range the music is set in (the highest and lowest notes in the song), and
(ii) Which notes should be played as sharps, flats or “naturals”.
Step back for a minute and think about how a typical “major scale” is constructed. It starts on Note 1 (the “root) and moves “up” in pitch a series of steps until it comes to Note 8 which is an octave higher than the root.
This piano keyboard gives us a graphic way to look at this. If you start on C and play a series of white notes you will come to C again, eight notes higher. These white notes represent what are called “natural notes” – C-D-E-F-G-A-B-C.
But what about thoses black keys?
But we also know that the piano has black keys. And some notes of the C scale have a black key between them. For example, there is a black key between C and D. So there is actually another note between the natural tones C and D, not part of the major scale. That note is called C# or Db**.
This happens because in the tradition of western (European) music, the smallest gap in pitch between successive musical sounds is called a “semi-tone”, or half a tone. But as we can see by looking at the piano, the gap between C and D is actually a full tone. There is another note a half tone above C, namely C# (or Db).
So far so good, but that’s not the end of the story. Still looking at the piano keyboard we can see that some natural notes (white keys) have a black key between them, and some don’t.
So it must be that some adjacent natural notes (white keys) are a full tone apart, and some are only a semi-tone apart (they don’t have a black key).
In fact, there are only two instances of this: E to F and B to C. These two gaps are only a semi-tone, or what is often called a “half-step”. All the other natural notes are separated by a full tone or “full step”.
OK, how is this relevant?
Not too surprisingly this is very relevant to the idea of musical keys.
When we say “This song is in the key of F”, for example, what we are actually saying is “Play the song so that the anchor note or root note is F”.
Try to understand this idea of an “anchor note”. Think of a song as a sequence of notes. That sequence of notes is like a grid placed on top of a bigger universe of all possible (singable or playable) notes.
Virtually all such songs have a natural focal or anchor point. That anchor point defines the starting and finishing point of the song and gives it a particular pitch range. If the anchor point is C we will play (or sing) one set of notes, centered around C. But if we move the anchor point up to D or E or G, the song will be centered at a different point and, as a result, the set of actual notes we sing or play will be different.
The gaps between the notes will be the same but because of the irregularity of the whole-step/half-step pattern, the non-natural notes (sharps or flats) will fall in different places.
Just to take a fairly simple example, the song “Ode to Joy” starts out: (where the number is the note of the major scale): 3-3-4-5-5-4-3-2. In the key of C this would be E-E-F-G-G-F-E-D, but in the key of D it would be F#-F#-G-A-A-G-F#-E.
How the musical staff helps simplify this
As we said at the beginning, changing the key of a song has two practical effects. First, it pushes the range of musical tones in the song either up or down.
And second, we end up playing some notes as sharps and flats, because the gaps between notes fall in different places.
It is this second point that affects the guitarist, pianist, or trumpeter the most. Because now it means some notes have to be played as sharps or flats; and the song sounds terrible if you screw that up.
What the musical staff allows us to do is to indicate that we are playing in a “key” where certain notes are to be played sharp or flat all the time (unless otherwise indicated).
If, for example, our anchor point is C (that is, if the song is played in the key of C) all the notes of the song will be natural (no sharps or flats) unless otherwise indicated.
But if we move the anchor point of the song up to D (that is, if the song is played in the key of D), the sequence of notes used in the song will include F sharp and C sharp. Every time we see one of these notes on the staff we know to play them as sharps. All the other notes are natural (unless otherwise indicated).
The use of the staff allows us to put the complicated musical theory aside and just focus on playing the correct notes.
*To keep it simple, C# and Db are the same note – the note between C and D. When we’re thinking in terms of sharps – e.g., in the key of D – we think of this note as C#. But when we’re thinking in terms of flats – e.g., in the key of Gb – we think of this note as Db. Yes it is confusing, but hang in there.