Watch the Melody: Horizontal vs. Vertical online guitar lesson by Matthieu Brandt from Songwriting on Guitar
In previous sections we talked about the pool of notes you can fish your melodies from. This pool is filled by adding all the individual chord tones of all the chords in your progression together. This will give you the options to create relatively smooth melodies, because they're often drawn from one key.
You can add to this pool of notes by introducing 'altered' chords - like the secondary dominants - 'mixed mode' chords and even other keys.
When you chose your melody notes from the pool generated by adding all the chord tones of your chord progression together, you are linking all the chords. With the melody you are glueing the chords together and you are creating what is called a 'vertical' melody. But each individual chord is a part of several keys and the notes from these scales all sound more or less good on that chord. This 'horizontal' way of looking at a chord - disregarding the chords that came before and the chords that follow - will add to the possible melody notes on that specific chord.