Another use for the chart should be to recognize your key signatures that are found on sheet music. Up near the clef (be it treble clef or bass clef) in the uppler left hand corner of the music staff, you will generally see a number of flat symbols (b) or sharp symbols (#). These signify the key in which the music is played. No sharps or flats means Key of C; 1 flat is Key of F, 2 flats, Key of Bb, and so on.
For example, if there are two sharps indicated in the Key Signature, what key are we in? Consult the column on the far right of the graphic. Yep, we are in the Key of D. In the rightmost column we see that the two sharps in the Key of D are F# and C#.
You can see in the chart which notes are sharp or flat for any of the twelve keys.
Here's another graphic on Key Signatures. I found this on the web at this location. This graphic shows also the minor key associated with the major key. (I'll explain later.)
C D E F G A B
The notes in the A minor scale are:
A B C D E F G
The Key of C and the Key of A minor have the same notes and are therefore harmonic equivalents of each other. Here's another example, chosen at random:
The A minor scale above is the Aeolian mode of the C major scale. It has a distinct minor sound to it. To play the Aeolian mode, just take the usual major scale and flat the 3rd, the 6th and the 7th notes of the major scale.
Notice the 3rd note in the A minor scale is a C. "A major" chord is A C# E (the 1, 3 and 5 of the 7 note scale). A C E is "A minor" chord -- the 3rd is always flatted in a minor chord, and C# becomes C.
The notes in the F major scale are F G A Bb C D E;
The notes in the corresponding minor scale of D minor are D E F G A Bb C.
Same notes, played in a different order. The first is sounds like the first scale we learned in grade school: Do Re Mi Fa Sol La Ti Do. The second has a spooky minor sound to it.
To find the minor equivalent of any major scale, go to the sixth note (or degree) of the major scale. The sixth note will name the minor equivalent. For example, in C scale, C, D, E, F, G, A, B, we see that A is the sixth note in C scale. Therefore, A minor is the harmonic equivalent of C major.