Editing Harmony (section)

== Chords and tension ==
{{Main|Chord (music)|Consonance and dissonance}}
In the Western tradition, in music after the seventeenth century, harmony is manipulated using [[chord (music)|chord]]s, which are combinations of [[pitch class]]es. In [[tertian]] harmony, so named after the interval of a third, the members of chords are found and named by stacking intervals of the third, starting with the "root", then the "third" above the root, and the "fifth" above the root (which is a third above the third), etc. (Chord members are named after their interval above the root.) [[dyad (music)|Dyad]]s, the simplest chords, contain only two members (see [[power chord]]s).

A chord with three members is called a [[Triad (music)|triad]] because it has three members, not because it is necessarily built in thirds (see [[Quartal and quintal harmony]] for chords built with other intervals). Depending on the size of the intervals being stacked, different qualities of chords are formed. In popular and jazz harmony, chords are named by their root plus various terms and characters indicating their qualities. To keep the nomenclature as simple as possible, some defaults are accepted (not tabulated here). For example, the chord members C, E, and G, form a C Major triad, called by default simply a C chord. In an A{{music|b}} chord (pronounced A-flat), the members are A{{music|b}}, C, and E{{music|b}}.

In many types of music, notably baroque, romantic, modern and jazz, chords are often augmented with "tensions". A tension is an additional chord member that creates a relatively dissonant interval in relation to the bass. Following the tertian practice of building chords by stacking thirds, the simplest first tension is added to a triad by stacking, on top of the existing root, third, and fifth, another third above the fifth, adding a new, potentially dissonant member a seventh away from the root (called the "seventh" of the chord) producing a four-note chord called a "[[seventh chord]]".

Depending on the widths of the individual thirds stacked to build the chord, the interval between the root and the seventh of the chord may be major, minor, or diminished. (The interval of an augmented seventh reproduces the root, and is therefore left out of the chordal nomenclature.) The nomenclature allows that, by default, "C7" indicates a chord with a root, third, fifth, and seventh spelled C, E, G, and B{{music|b}}. Other types of seventh chords must be named more explicitly, such as "C Major 7" (spelled C, E, G, B), "C augmented 7" (here the word augmented applies to the fifth, not the seventh, spelled C, E, G{{music|#}}, B{{music|b}}), etc. (For a more complete exposition of nomenclature see [[Chord (music)]].)

Continuing to stack thirds on top of a seventh chord produces extensions, and brings in the "extended tensions" or "upper tensions" (those more than an octave above the root when stacked in thirds), the ninths, elevenths, and thirteenths. This creates the chords named after them. (Except for dyads and triads, tertian chord types are named for the interval of the largest size and magnitude in use in the stack, not for the number of chord members : thus a ninth chord has five members ''[tonic, 3rd, 5th, 7th, '''9'''th]'', not nine.) Extensions beyond the thirteenth reproduce existing chord members and are (usually) left out of the nomenclature. Complex harmonies based on [[extended chord]]s are found in abundance in jazz, late-romantic music, modern orchestral works, film music, etc.

Typically, in the classical [[common practice period]] a dissonant chord (chord with tension) ''resolves'' to a consonant chord. [[Harmonization]] usually sounds pleasant to the ear when there is a balance between the consonant and dissonant sounds. In simple words, that occurs when there is a balance between "tense" and "relaxed" moments. For this reason, usually tension is 'prepared' and then 'resolved',<ref name="Schejtman">Schejtman, Rod (2008). ''The Piano Encyclopedia's "Music Fundamentals eBook"'', pp. 20–43 (accessed 10 March 2009) [http://PianoEncyclopedia.com PianoEncyclopedia.com]</ref> where preparing tension means to place a series of consonant chords that lead smoothly to the dissonant chord. In this way the composer ensures introducing tension smoothly, without disturbing the listener.<!--this is a contradiction--> Once the piece reaches its sub-climax, the listener needs a moment of relaxation to clear up the tension, which is obtained by playing a consonant chord that resolves the tension of the previous chords. The clearing of this tension usually sounds pleasant to the listener, though this is not always the case in late-nineteenth century music, such as ''[[Tristan und Isolde]]'' by Richard Wagner.<ref name="Schejtman"/>