For example, if I’m playing a song in G major on the piano, accidentally playing F natural immediately sounds off, but when the composer indicates to play natural, it still flows. What’s the difference?
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**ELI 5 version**
The perception of a note being “good” or “off” in a key signature depends on its relationship to the key, its role in the harmonic context, and the musical intent. Musicians often experiment with non-diatonic notes to create tension, express emotions, or add unique elements to their compositions. When used skillfully, these notes can enhance the musical experience, but when used haphazardly, they may sound discordant or out of place.
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Key signatures establish the tonal center, or key, of a piece of music. In a key signature, certain notes are marked as natural, sharp, or flat by default. For example, in the key of C major, there are no sharps or flats, so all the notes are natural. In contrast, in the key of G major, F is sharp (F#). Following this, D has two sharps, A has three, then E, B, F#, and C# (all major).
“**Go Down And Eat Breakfast Frank Sharp**” is the pneumonic device I use. Zero accidentals is C Major. All 7 accidentals is either C# Major for sharps and C♭Major for flats.
Also, for flats it’s the circle of 4ths instead of 5ths. Starting with the key of F major, which has one flat, the pneumonic device goes:”**Fanny Baker ♭, Eats ♭, Apple ♭, Dumplings ♭, Greedily ♭**”
Which sharps are added? Just remember this like it’s a telephone number: **FCGDAEB**, orin other words: **F# C# G# D# A# E# B#**
Which flats are added? The reverse order – **BEADGCF**,or i.e.: **B♭ E♭ A♭ D♭ G♭ C♭ F♭**
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**Deeper explanation version**
Harmonic Context is crucial here. When a note is played, its relationship to the key’s tonic (the first note of the scale) and the underlying chords and harmony in the music are crucial. In some cases, playing a note that is not in the key signature can create a sense of tension or dissonance, which can be used effectively for expressive purposes. In other cases, playing such a note can sound dissonant and unpleasant.
The tension created by playing a non-diatonic note is often resolved by moving to a diatonic note that is part of the key signature. The resolution can make the initially “off” note sound good in the context of the overall musical progression. In your example of G major, flatting the sharped 7 (i.e. playing F instead of F#) is a key modulation that changes to a key signature with one fewer sharp. The G7 chord went from being a major 7 to a dominant 7 as it now functions as the dominant V to a new tonic: C. And the key of C has zero sharps, one fewer than G’s single sharp on F. Does that make sense?
This is known as a secondary dominant (aka: applied dominant, artificial dominant, or borrowed dominant). It’s a major triad or dominant seventh chord built and set to resolve to a scale degree other than the tonic, with the dominant of the dominant (written as V/V or V of V) being the most frequently encountered. The chord that the secondary dominant is the dominant of is said to be a temporarily tonicized chord. The secondary dominant is normally, though not always, followed by the tonicized chord. Tonicizations that last longer than a phrase are generally regarded as modulations to a new key (or new tonic).
Typically it sounds best to modulate to a key only one accidentally away, either in the flat direction of the sharp direction, so to speak. So to move one accidental flatter, take your I chord and make it a V7 chord (that’s V7/IV), resolve it to IV as a new tonic. To move the key sharp, make your minor ii chord a major II chord by sharping to a major third in the ii to II triad, and add the 7 to make it a dominant V again. E.g.: to get from a C Maj tonic back to a G Maj tonic, go to C Maj’s ii chord: D-F-A, except make the F an F# and add the C to make it a dominant V7 chord. C dominant V7 of course resolves to a tonic of F.
According to music theorists David Beach and Ryan C. McClelland, “[t]he purpose of the secondary dominant is to place emphasis on a chord within the diatonic progression.” The secondary-dominant terminology is still usually applied even if the chord resolution is nonfunctional. For example, the V/ii label is still used even if the V/ii chord is not followed by ii.
More reading: [https://en.wikipedia.org/wiki/Secondary_chord](https://en.wikipedia.org/wiki/Secondary_chord)
Some intervals are more dissident than others. The order from least to most dissident is as such
Octave X
Major 5th. X
Major 4th. X
Major 3rd. X
Minor 6th
Minor 3rd
Major 6th. X
Major 2nd. X
Minor 7th
Minor 2nd
Major 7th. X
Tritone
The one with X is in the western major scale. Notice how they’re on the less dissident side. Including a minor 3rd or Minor 6th in a melody wouldn’t sound too out. Adding the Tritone would feel quite jarring.
Edit, with your example, an F is the Minor 7th in G. These can sound good if you make a Gm9 chord with G Bb D F A for example. Also a dominant G7 chord would sound good whilst resolving to C
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