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In wikipedia page - jazz chords : Major Seventh it is said that "if the melody note is the root of the chord, including a major seventh can frequently cause a harsh dissonance". But since the root note itself is played in the melody, what is the reason for the dissonance? (of course there is a harsh dissonance when played so)
Are there any other such principles ?

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Even if the b2 interval mentioned in Dan Davis's answer is avoided by using a different voicing, the problem that is usually meant in this context is the b9 interval between the major 7th and the (higher) root note. The b9 interval is considered a very dissonant interval which in traditional jazz harmony is only "allowed" on a dominant seventh chord functioning as a V chord (e.g. G7(b9) resolving to Cmaj7).

Having said all that, that's only the theory, so you shouldn't worry about it too much. Listen to great players and you'll notice how often they use that sound. I've seen and heard Allan Holdsworth play a major seventh chord in this voicing (from low to high):

5th 7th 3rd root

So here you also get a b9 interval between the low major seventh and the high root. No problem, let your ears decide.

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    I guess Holdsworth absolutely hated the sound of a major seventh chord with the seventh on top. I believe he referred to it as "ugly", but I can't seem to find the video I saw it in. – Dan D Mar 23 '15 at 19:12
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It looks like that bit of information has been in the article since it was written. From the original 2005 article:

Often the melody note or other pitched phenomena influences which of the above chord types a performer selects. For example, if the melody note is the root of the chord, including a major seventh can frequently cause a harsh dissonance.

I can only assume what the author may have meant. In music in general, lower harmonies are less forgiving of dissonances. As a result, intervals between lower notes generally have to be more consonant. I'm pretty sure this is because the frequencies in lower octaves are closer together, but I don't know of any studies to prove this. Only theory. The implications of this in jazz are that harmonic extensions will most likely go higher in the chord than lower. So the logic would go that if the melody, usually containing the highest or most prominent note, is also the root of the chord, then the higher notes (extensions) are more likely to be dissonant than the lower ones (main chord tones). This would be especially true if you were using the simplistic voicing pictured:

Major Seventh Chord

Another C on top of that chord could be jarring. Honestly, I don't mind this sound, but I can see how it would be considered dissonant. While the major 7th interval isn't the best sounding interval on its own, the perfect 5th it makes with the 3rd of the chord and the minor 3rd it makes with the 5th of the chord make it a wonderful note in the chord. However, the minor 2nd that would occur if the root were directly above it and the resulting minor 2nd is going to be much more harsh on the ears. I think this is what the author intended. There's a lot of assumptions and general tendencies there, but it gets the point across.

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Spacing ("voicing") a chord like that makes the interval between the topmost "seventh" and the melody note a semitone, also called a minor second. A different spacing would change that to a major seventh.

Minor seconds sound harsher than major sevenths, because the notes of a minor second usually occupy the same psychoacoustic critical band. That's why, for example, Pierre Boulez's meticulous integral serialism spaces chords (ok, "pitch simultaneities") by avoiding minor seconds.

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A minor 2nd interval (two notes that are one half-step apart) is used in the chord. Minor 2nds generally sound dissonant and not very good. On a piano, try playing B and C or F and F# together.

The major seventh chord doesn't sound quite as dissonant as this because the B and C are in different octaves, but it's definitely not as pleasing as a regular major chord.

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