If you start to transfer music from fake books to MMA you'll find
chords which look like C/E, Dm/C or Am/C. These chords are called
"slash chords".

Unfortunately, the notation used by arrangers and composers who
create fakebooks and lead sheets in not nearly as standardized as one
might like. And, when it comes to slash chords, the standard is far
from firm. So, what to do in MMA?

Let's look at the different ways that slash chords are used, but
first lets revisit the method used by MMA to create a chord. MMA has
a set of tables which represent each type of chord in terms of MIDI
note values. These note values are adjusted for the particular scale.
For example, the "notes" for a Major chord are stored in the table as
(0,4,7). If you specify a F Major chord the notes will be converted
to (5,9,12), which corresponds to F,A,C. In addition, each chord has
a list of notes for a corresponding scale. A Major chord will have
the notes for a Major scale, etc. So, in the previous example, a
scale list (5,7...16) representing the notes F,G,A,Bb,C,D,E is also
created.

So, on to the slash chords ... we'll show different types and what MMA
does and doesn't do.

The easiest case is the simple inversion. In this case the slash note
will be a note in the chord. Examples include Am/C, A/C#  and C/E.
MMA handles these by:

 - Rotating the chord so that the slash note becomes the root. An
   Am/C chord (A,C,E) becomes C,E,A; the C/E chord (C,E,G) becomes E,G,C.

 - Rotating the associated scale. In the case of Am/C the scale
   A,B,C,D,E,F,G becomes C,D,E,F,G,A,B.

This may OR MAY NOT effect the output, especially if you have enabled
MODE=OPTIMAL which may do chord rotation on its own. In most cases the
bass notes will be effected.


The next case encountered is a slash note which is not in the chord,
but is in the scale. Example of this include C/B and Am/F. A C Major
chord does not have a B note in it (although a B Major 7th chord
does), but the scale associated with the C Major chord C,D,E,F,G,A,B
does. So, MMA leaves the chord alone and rotates the scale to
B,C,D,E,F,G,A. Scale lists are used for bass and scale patterns.
Since scale patterns are mostly circular, the effect on these
patterns is negligible; however, for bass patterns the root note (1)
will be changed from C to F ... which is probably what you want.
However, note that the 5th is also changed from G to F which might
NOT be what you want.


Yet another slash notation is to have the slash note as neither a
scale or chord note. For example, one might want a C chord with the
dominant 7th played ONLY in the bass. The notation for this would be
C/Bb. If you encounter this, just change the chord to C7 and you'll
probably be as close as you're going to get. We've played with
different methods of handling this and haven't come up with anything
satisfactory. We considered the following (all of which ignore the
chord notes):

 - Just adding the Bb note to the scale. But, that's quite dumb since
   in the case of C/Bb we'd end up with a scale Bb,C,D,E,F,G,A,B. Not
   only is this 1 note too long (all scales are 7 notes long!) it has a
   B and Bb in it.

 - Add the note and drop the extra note. In the above case this gives
   us Bb,C,D,E,F,G,A. Perfect! But, what if we have C/Eb? If I saw this
   in a chart I'd probably figure that we want a Cm chord with the Eb
   played only in the bass. But, really, it could be just about
   anything. And, if we followed the add and drop routine we'd have
   Eb,C,D,E,F,G,A,B, which leaves that nasty E.

 - Find a "close" note, substitute and rotate. Well, not a bad idea.
   In the case of C/Bb we could parse the scale and discover that B is
   only one MIDI value away from Bb. Okay, lets change the B to a Bb and
   rotate. Perfect ... but, the note A is only one value out too. So,
   maybe that's the one to modify. If we had the tables listed as note
   names instead of MIDI values this might be simpler. But, as it is ...
   well, it's just not going to be reliable.

So, in this case MMA just reports a failure, but doesn't barf.


In some charts (not so much new ones, but you never know) you'll see
things like Cm/9 or even Cm/dim. One can only guess at the meanings
... and MMA avoids guessing. So, in these cases it just reports an
error.



bvdp, December/04