Understanding key signature in garage band

Question

I am relatively new to understanding key signatures.I understand the basic difference between scales and key signatures. I have come across the following set of notes for C major key signature in garage band under smart guitar section

Em Am Dm G C F Bb Bdim

Aren't the notes for C major key signature same as C major scale which are C D E F G A B ?

Can someone explain me the reason why the notes in C major key signature in garage band ios app different ?

Answer 1

Em Am Dm G C F Bb Bdim

these are chords that are (with the exception of Bb) diatonic to the key, meaning they are chords that are made from notes in the key of C (the c scale notes).

for example Em is E minor, or an E minor triad and contains the notes E, G, B.

C is C major, C, E, G.

It is also common to assign numbers to these chords' root notes. So C is I (roman numeral 1), F is IV, G is V and are all major triads.

ii, iii, vi are all minor (D, E and A minor triads).

vii dim is a diminished chords.

Bb is a chord that is built on the flatted 7th degree of the scale, it is borrowed from another scale.

All major keys will have a I, ii, iii, IV, V, Vi, viio set of chords (note upper case for major, lower for minor, a little circle like a degree symbol for diminished) that are made from diatonic notes to the scale. this is why the roman numerals are useful. If a song has a progression of I, V, vi, IV you can play those chords in any key and the relationship between the chords will remain the same. you fingers will need to change what they are playing but the progressions functional harmony will remain the same. this is called transposing, or changing keys with out changing the relationship of the chords in context.

Answer 2

Garageband bases its default chords for the Smart instruments off of common chords used in that key, regardless of whether the chord is diatonic or not. So, Garageband chose Em, Am, Dm, G, C, F, B♭, and Bdim. These chords are used the most often in C major, and when changing the key in GB, you'll find that these same chords get transposed up or down the same amount. If you wish to change the chords, you can do so manually, but I forget how.

If you'd like an explanation of the functions of the chords, here they are:

• Em = iii (diatonic, tends to go to IV or vi)
• Am = vi (diatonic, has tonic function)
• Dm = ii (diatonic, subdominant function and tends to go to V for ii-V, common in jazz)
• G = V (diatonic, dominant function and stringest resolution to I)
• C = I (diatonic, is the tonic for the song)
• F = IV (diatonic, subdominant function, tends to go to V or sometimes straight to I)
• Bdim = viio (technically not entirely diatonic if made a 7th chord but it is the most common diminished chord in C major, usually resolves any of its 3 notes up by a half step)
• B♭ = ♭VII (borrowed from the minor mode, this chord is common in pieces that stray from diatonic harmony, and it can have subdominant function).

Bonus fact: The most immediately confusing part of this is just the weird order the chords are in. The order is such that going from left to right produces a circle progression, as all the chords are a perfect fourth away from their neighbors (except the Bdim and B♭, which were probably both included to combine diatonic simplicity with common variations.