How are melodic intervals different from harmonic intervals?

In music, an interval represents the inclusive distance between two tones. The primary difference between melodic and harmonic intervals lies in how these tones are sounded:1

• Melodic intervals consist of two tones sounded successively, one after the other. These intervals form the horizontal component of music, contributing to the linear sequence of pitches that we perceive as a melody.12
• Harmonic intervals consist of two tones sounded simultaneously. These intervals represent the vertical component of music and serve as the building blocks for harmony and chords.13

While they differ in their temporal presentation, both types of intervals are measured by the same relationship to diatonic scales, counting the lines and spaces between notes on a staff. In Western tonal systems, both melodic and harmonic intervals are often categorized by their quality—such as perfect, major, minor, augmented, or diminished—and their perceived level of consonance or dissonance.1