## hpr1754 :: D7? Why Seven?

 In this episode I respond to one of the community-requested topics ("Music Theory") and try to explain what seventh chords are and why they are used. Below are some of the terms that I use in the course of the discussion.

Interval: The distance between two pitches (sounded either consecutively or simultaneously)
Consonance: Relatively stable sound between two or more pitches
Dissonance: Relatively unstable sound between two or more pitches. Dissonance often needs a "resolution" to consonance
Chord: three or more notes sounded together 
Chord progression: a succession of chords 
Triad: a chord with 3 pitches, the adjacent pitches separated by the interval of the 3rd.
Seventh chord: a chord with 4 pitches, the adjacent pitches separated by the interval of the 3rd.
Tonality: harmonic system that governs the use of major and minor keys
Tonic: the central tone of a piece of music
Mode: major or minor [e.g. Symphony no. 5 in C minor]
Modulation: the process of changing keys within a piece of music 
Scale: Ascending or descending series of notes that define a key or tonality, with a specific arrangements of half-steps and whole-steps. Major and Minor scales are most common in Western music

Free public-domain music reference book: Music Notation and Terminology by Karl Wilson Gehrkens: https://www.gutenberg.org/ebooks/19499 (see ch. 18)
Free Online Music Dictionary: https://dictionary.onmusic.org/