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Key signature distinction between major and minor chords

Posted by jonel 
Key signature distinction between major and minor chords
August 03, 2018 12:19PM
Hi All,
Every major chord has a relative minor that uses exactly the same keys and chords. So is there anyway to determine if a melody is in say A minor or C major key just by looking at the key signature. I'm sure it's probably a daft question but the more I think about this the more I need to ask.

Thanks
ttw
Re: Key signature distinction between major and minor chords
August 06, 2018 08:35PM
The key signature alone is not sufficient. One could (and some have) written tonal music with no key signature. (Or look at the history of horn notation over the centuries.) At one time, during part of the Baroque Era (and perhaps contributing the name)it was common (at least for flat keys) to use a key signature of one fewer flat. (Cm would have 2 flats and Dm none; supposedly needs fewer accidentals. One swaps the need to put in an Ab with the ability to leave out the B-natural. Seems like almost an even trade off to me.)

To determine the key of a region of music, one must look at the emphasized cadences at the end of sections mainly. The last cadence usually suffices. Of course a piece may modulate and not return (marches and polkas often have a trio in the subdominant and also often to not return to the main key.)
Re: Key signature distinction between major and minor chords
August 07, 2018 01:34AM
Thank you so much for responding. I'm not really sure I understand your method of determination (but that is my own level of understanding music). My notion of at least recognising a minor key on sheet music was to look at the number of accidentals in the notes.The major key should have few or no accidentals.

Jonel
ttw
Re: Key signature distinction between major and minor chords
August 07, 2018 06:24AM
Guessing the key from the number of accidentals is not too bad but there are highly chromatic pieces which use lots of non-diatonic notes. It's really the final chords. Note that "key" is a concept that may be applicable to a shorter span than an entire piece or even part of a piece. "Key" is a fuzzy concept in that pieces with a clear key center are easily identifiable but some pieces are on the edges.

There are few tricky things. One may have a "secondary dominant" (also called an applied dominant or attendant dominant or the like) where a single chord is preceded by its dominant seventh. In a piece in C major, the sequence C, D7, G7, C is not uncommon. The D7 to G7 sounds a lot like a cadence but its effects only last two chords. The name given is "secondary dominant" (I guess this is as good as any name, it's a dominant formed from a secondary chord, D minor in this case, in the key) and is quite common. Both C, d, G7, C and the above occur even in the same piece. Often a piece will have a short phrase ending on other than the tonic. One may see pieces in C with a phrase ending with a G chord or an A minor chord or whatever. These are good candidates for a secondary dominant treatment. (One can even write something like: b, E7, a followed by d, G, C. The first phrase is in "a minor" but it's so short that the whole phrase probably sounds like it's in C with slight variation.)

Minor keys have two mutable notes. No matter the key signature, there will be some accidentals.
Re: Key signature distinction between major and minor chords
August 07, 2018 07:17AM
Thanks a lot. I think I have a lot of work to do! Just when I thought I was getting on top of it - lol.
Re: Key signature distinction between major and minor chords
August 14, 2018 06:21AM
As has already been said, the answer to the question is no.

What hasn't been mentioned yet though is a quick and easy way check is to look for the leading note of minor keys, that is the raised seventh degree (which typically moves up to the tonic). So for a piece with no sharps or flats in the key signature, the two likeliest keys are C major and A minor. Look for any G#s in the piece, if there are several of them moving up to A, it's a good betting the piece is in A minor. If there are none, it's much more likely to be C major.
(For a key signature of one sharp, look for the presence of D#s that might indicate E minor. For a signature of one flat, look for C#s that might indicate D minor, and so on)

Of course things do get more complicated than than (and it doesn't always work) as ttw has already touched on, but this is often a useful starting point.

Check out this textbook on Four Part Harmony.
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