The quality or state of being harmonious; the presence of harmony, accord, and pleasing agreement.
From harmonious (harmony + -ous) + -ness (abstract noun suffix). This double-suffixed formation creates an abstract noun representing the essential quality of harmonic agreement.
The 'harmoniousness' of a painting, a piece of music, or a group of people is instantly recognizable to humans—our brains are wired to perceive and prefer balanced, proportional, non-conflicting arrangements, which is why we call it beautiful.
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