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According to Wikipedia, a chord is

any harmonic set of two or more notes that is heard as if sounding simultaneously.

But I always thought that two notes aren't a chord, they are an interval. And according to the Intervals Wikipedia page, an interval is

the difference between two pitches.

So my question is this: Can two notes be considered a chord? And why?

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An interval in this case is an instrument of measure and the word chord names the item. The notes in a chord are separated by intervals. Lots of people consider 2 notes together a chord fragment and not a true chord and call it a diad where others consider the minimum chord a triad (3 notes). Very awkwardly phrased, sorry... – JimR Dec 25 '13 at 8:09
Chords with 3 or more notes all have names - some rather weird looking at earlier posts ! We're probably aware of the 5th chord (power chord to guitarists), but what names would be attributed to other diads ? Presumably reflecting the interval they use ? This potentially could get confusing - is C6 just C+A, or the original C+E+G+A ? – Tim Dec 25 '13 at 10:29
(To confuse things further :) -- In alot of the fake book stuff I've seen, the typical Chuck Berry style shuffle (Think Johnny B. Goode) calls the alternated chords, in the key of C, C and C6. – JimR Dec 25 '13 at 17:17
up vote 12 down vote accepted

An interval is the difference between two pitches regardless of whether they are played together or one at a time.

A chord is a combination of notes played simultaneously.

Just to confuse matters, some sources define a chord as having three or more notes (personally I call two notes a chord).

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Note there is a subtly different thing called a "simultaneity," which Wikipedia defines as "more than one pitch or pitch class all of which occur at the same time, or simultaneously. Simultaneity is a more general term than chord: most chords or harmonies are then simultaneities, though not all simultaneities are chords." ( – Adrian Holovaty Dec 31 '13 at 20:53

Personally, two pitches are not sufficient to define a chord because it is incomplete. Three is the minimum. When you press two strings in a guitar, it may produce some sort of a chord but it is not complete. For instance, when we play a power chord which consists of the root note and the fifth interval, it could be either a major chord or a minor chord. It can only be known based on other chords used along with it.

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I agree with you on your answer. Two notes cannot establish a chord quality by themselves without a bass note or any other note to identify it being major, minor, diminihsed, etc. – r lo Feb 21 '14 at 13:05
Not true at all. You can have a root and a 3rd and omit the 5th on most triads because the 5th is implied. in fact, it is a common practice in voice leading if the complete chord would lead to poor voice leading to omit the 5th. – Dom Feb 21 '14 at 15:46

An interval is a measurement; a way to analyze music theoretically. A chord is an actual musical component. If you "play intervals", you could technically be said to be playing arpeggios, which are "broken chords".

Many definitions of a chord require a third note, drawing from Euclidean geometric definitions: "two points define a line; three define a plane, four define space". The parallel definitions would state that two notes define movement of pitch, but three are needed for a chord in order to define its quality (major, minor, suspended, augmented, diminished). However, two notes, especially two lines of notes, can indicate the quality of a key in which the performers are singing or playing.

For example, listen to this: There Is No Rose - Traditional Carol

The full choir is in more or less three-part harmony; though there are four parts, two or more double on each sustained chord. But then the middle portion drops to a duet. With the tonal quality of the piece defined by the fuller harmony, the movement of the soloists relative to each other implies the same tonal quality even though the third note, which would explicitly define that quality, isn't there.

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