The costrunction of a chord is based on a root note and a structure,
so by default, giving just a note, it creates a major chord adding the
third and fifth note.
Quite some time ago I wrote something to solve the reverse problem:
given a list of note names, find out a chord name.
(http://chordrecognizer.sourceforge.net/ )
I used a look-up table to made a correspondence between the chord-name
and the notes from which the chord is built. The notes are expressed as
a distance from the root note.
E.g. starting from a chromatic scale built on C, the distances would be:
C:0 C#:1 D:2 D#:3 E:4 F:5 F#:6 G:7 G#:8 A:9 A#:10 B:11
(and similar for flats, double sharps and double flats, ...)
Any chord thus can be constructed from a root note + the distance
information.
example distance information:
{ 'm' : [ 0, 3, 7 ], # minor triad
'' : [ 0, 4, 7 ], # major triad
'7' : [ 0, 4, 7, 10] # etc...
...
}
How does one e.g. construct the E7 chord from this information?
1. generate the chromatic scale starting from E, and annotate with note
distance to root note:
E:0 F:1 F#:2 G:3 G#:4 A:5 A#:6 B:7 C:8 C#:9 D:10 D#:11
2. take the recipe for a '7' chord from the distance information:
[0, 4, 7, 10]
3. map the numbers from step 2. to the note names from step 1.: E G# B D
If you care about proper enharmonic spelling of the chord's note names
(i.e. do not confuse F# and Gb), you will need to add some extra
information in the look-up tables or you need to pass extra information
to the chord construction recipe at the moment of creating a chord, but
that is left as an excercise to you - the interested reader ;)
HTH,
Stefaan.
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