Watch the Melody: Light and Jazzy online guitar lesson by Matthieu Brandt from Songwriting on Guitar
Using the upper extensions of a chord will give the melody a lighter, even jazzier sound. Targeting the major seventh, ninth, eleventh or thirteenth on top of a major or minor chord means your melody will float. If these melody notes sit within the key you are using, the resulting floating sound will be jazzy. If you chose to add notes that can not be found within that key, the resulting melody will create tension.