ChordPatterns


 * Note numbers in parentheses are optional **

Intro
This is a complete reference on chord formation (from here).

This is a helpful chord tree that will aid in the writing and analysis of music. You may move down the tree but cannot retrograde backwards until reaching the bottom. There are, of course, exceptions including cadencing using the IV to I and the so-called deceptive cadences that typically go from V to vi. You may move across the levels and interchange the chords and can skip levels entirely if so desired. (Example- I, IV, ii, V, I, vi, V, I)
 * {|style="text-align:center"


 * I
 * vi --- iii
 * IV --- ii
 * V --- vii7
 * I
 * }
 * V --- vii7
 * I
 * }
 * I
 * }

Dominant/Seventh
Normal

The dominant seventh is sometimes just called the "seventh", even though it contains a minor seventh and not a major seventh.

Altered

Suspended

Any chord with a major third can have the third replaced by a major second or perfect fourth to form a suspended chord. Given a chord C, this would be written Csus2 ("C suspended second") and Csus4 ("C suspended fourth") for a major chord. As these only apply to the major second and major fourth, there is no such thing as, say, a Csus6, which would probably be properly written as Gsus4/C or C5add6 if it has a fifth, or else Am/C. However, in certain circles it is said thought that replacing the fifth in a triad with a sixth is a type of suspention, eg Csus6 would contain C, E and A, rather like an Am. This is however more accurately notated as C6.

Minor
Normal

Diminished

Notice the diminished seventh chord has a double-flatted seventh, which is enharmonically the same as a sixth, so you may find the diminished chord alternately written as "1 b3 b5 6". The double-flat notation is more correct because it shows how the seventh is further diminished from the half-diminished state.

Sometimes a diminished seventh chord is notated the same way as a diminished triad. Even more confusingly, sometimes both forms are called the "diminished chord"! You can make the distinction between the two to avoid ambiguity. However, keep in mind that they're ultimately very similar chords anyways, and you can usually interchange them.

Other
It's worth mentioning that you might hear some people (usually rock guitarists) call fifth chords 'power chords'.

9th, 11th, and 13th chords
7th chords can be extended to 9th, 11 and 13th chords. If you have a C7 (C dominant seventh), then the corresponding chords would be C9, C11 and C13. The C9 is a C7 with a major ninth (or second) added. The C11 is a C7 with a eleventh (or perfect fourth) added. The C13 is a C7 with a major thirteenth (or sixth) added.

The same principles that can be applied to seventh chords apply to minor and major 7th chords. For instance, you can have a minor 13th: 1 b3 (5) b7 13. The notes added are always the same: ninth, eleventh, and/or thirteenth, not augmented or diminished. Only the seventh chord base changes.

It is very uncommon to extend a major 7th chord with an 11th. This is because there would be an interval of a minor ninth between the third and the eleventh. If an eleventh is played, it is usually altered (this is dealt with below).

Additions
When a note is added to these chords in a manner that does not fit the tables above, it is often written "addX", where X is the number of the added note, e.g., add6 for an added sixth. For instance, C major with an added sixth would usually be written Cadd6. If the number is above 7, then it will be an octave higher than the root note.

Do not confuse Csus2 and Cadd2, or Csus4 and Cadd4. Suspended chords must not have a major third, but major chords with additions must. (Note that in jazz, suspended chords sometimes have a major third added, but only when the player wants dissonance).

Do not confuse Cadd9 with C9; the latter has a minor seventh, but the former does not.

An "added second" and "added ninth" are often considered synonymous, because a ninth is a second. Some musicians argue they imply different voicings of the chords, for instance, add9 should have the second raised an octave, but add2 should not. The "added ninth" is more common, especially since it is rare that one wants to add a major second that is literally one whole tone away from the first (and possibly a third as well).

The 9th here is what is often referred to as a compound interval, that being an interval greater than an octave, such as a 9th being an octave up from a 2nd, a minor 10th being an octave up from a minor 3rd... Greater intervals eg a minor 17th theoretically exist (this being a minor 3rd raised 2 octaves) but such things above a modification of a 13th are never spoken of. To create a compound interval one would add 7 (once for every octave) raised to the number in the name to raise the interval. The same applies subtracting seven to lower the pitch by octaves.

Alterations
Sometimes something such as "b5" or "#5" (also written "-5" and "+5", respectively) will be appended to the end of the chord; this means play the chord as normal but flatten or sharpen the fifth respectively. (This is why C&oslash; is sometimes written Cm7b5, because a minor 7th with a flatted fifth has the same notes as a half-diminished chord. Similarly, it may also be pronounced "C minor seventh flat five".)

Any of the extension notes (9th, 11th and 13th) can also be altered, something fairly common in jazz. The possible alterations are b9, #9, #11 and b13. You may be wondering why there is no b11 or #13. Stop and think for a second about which notes they would be in the key of C major. The b11 would be E, which is the major third in C, and the #13 would be Bb, the minor 7th, so a chord symbol like C b11 #13 would imply C7. Which would you rather read?

The table below shows common altered chords:

The names in brackets after the full name refer to the mode from which the chord is derived (there should be a section on this eventually).

Bass notes
The bass note of a chord is the lowest note of the chord. The most common case is that the bass note is the root note, for instance, in a C major chord C-E-G ascending in pitch from left to right, the bass note is C, which is the root note. But if the chord were instead G-C-E, it is still a C major chord (specifically, an inverted C major), but G is the bass note, while C is still the root note. Sometimes the bass note is omitted for brevity when it is still a part of the chord, which may be needed, for instance, when many rapid chord changes would otherwise make the names illegible on a printed score.

Occasionally, bass notes are not a part of the original chord. For instance, a D minor does not have a C note, but sometimes Dm/C is seen, meaning the D minor chord is played normally but with a C note below it. These are often called slash chords.

This notation can also indicate a polychord.

Polychords
Polychords are superimposed chords. They are more common in some kinds of music (jazz) than others (classical).

This may occur, for instance, when a piano player plays a D minor with the left hand (possibly omitting its fifth) and a G major with the right hand. The most common notation for this polychord is probably G/D. This leaves out something important: that the D is a minor chord! (Usually, only the first chord, the higher one in pitch, has anything more than a single note specified.) This also clashes with the more common meaning of "G chord but with a D bass note", which is really a specific case of a polychord where the lower "chord" has only one note. Therefore, this notation is incomplete, but it is the closest thing to a standard notation for polychords. There are many nonstandard systems in use, however. Sheet music using polychords should explain the system used, if possible.

A common use of polychords is arpeggiating one chord against a different chord. For instance, on a piano, the left hand may be holding down a G5 chord while the right hand arpeggiates D minor chords. This would be notated Dm/G.

Polychords might be used as exotic voicings for many chords. The following examples use a pipe (|) to separate the upper chord from the lower one. (the examples are given here for a C chord)

Quartal Chords
Some arrangements use chords based on fourths, often two fourths in the upper notes with an independent bass, which gives the following possibilities:

Of course, the fourths part of the chord can be present in two other inversons - sus4 or sus2, making Bbsus47/C, Ebsus4/C and Absus2/C equivalent for instance.

Chord implication
Sometimes chords are not specifically indicated by notes, but are indicated by the chord structure, or even notes that don't form chords by themselves but sound like they belong to a certain chord. The most usual way to handle this is to put the chord name in parentheses. Implied chords are also often used when there are constant chord changes throughout a section (several per measure), and marking each one would make little sense, so the "underlying" chord is chosen (usually a chord the others are temporarily centered around). Sometimes sheet music will use only implied chords, in which case often chord names will appear normally, and perhaps a footnote will state something such as "chords reflect implied harmony".