Mozart, who had not yet met Haydn, under the shock of the discovery of the so-called "sun" quartets, wrote in quick succession in 1773 his series of six so-called "Viennese" quartets, he is 17 years old.
 
All biographers agree that it was Haydn who sealed the rules of the string quartet: the composition of the ensemble? two violins, a viola, a cello; the overall balance? the four instruments converse, dialogue, communicate or confront each other but are always expressed on an equal footing. Haydn limits the number of movements in a quartet to four.
 
Goethe wrote[15] "We hear four intelligent people conversing among themselves, we think we capture pieces of their conversation while discovering something specific to the instruments". This metaphor nicely captures two defining features of the genre: its intimate, personal nature as well as its capacity to convey profound musical thought through the essential ingredients of harmony and counterpoint[16].
 
It was the discovery of the Opus 33 quartets called "Russian Quartets" composed by Haydn in 1781, nine years after Opus 20, that again inspired Mozart. Like his elder, Mozart had abandoned the genre for 9 years. Deeply impressed by Haydn’s Op 33 quartets, Mozart wanted to rise to the level of his model. He composed the quartets dedicated to Joseph Haydn from 1782 to 1785 which are, by his admission, the fruit of a long and arduous work.
 
Seven months after Papa Haydn’s compliment on Wolfgang[17], the Viennese publisher Artaria published the six “Quartets dedicated to J. Haydn”. This publication will be accompanied by the famous and moving dedication of Wolfgang to his friend:
 
“To my dear friend Haydn
A father who had decided to send his sons out into the great world thought it was his duty to entrust them to the protection and guidance of a man who was very famous at that time, and who happened moreover to be his best friend. In like manner, I send my six sons to you, most famous man and very dear friend. They are, indeed, the fruit of a long and laborious work… Please, then, receive them kindly and be to them a father, a guide, and a friend! ...
I entreat you, however, to be indulgent to those faults which may have escaped a father's partial eye, and in spite of them, to continue your generous friendship towards one who so highly appreciates it.
Meanwhile, I remain with all my heart, dearest friend,
Your most sincere friend.”
 
We recommend listening to the quartet n ° 19 K 465 in C minor "Les dissonances" (especially the first movement - Adagio)[18]. Obviously, we hear dissonances, strange friction of notes, which, when we do not know the work, leaves us wondering about its author. Some well-meaning musicians were tempted to correct the anomalies from which it suffered and, although he himself was quite troubled, Haydn defended Mozart by saying: "He must have his reasons".

 
 
III - "Monday Mozart composes like Haydn, Tuesday Haydn composes like Mozart".

Montaigne's most famous phrase will be remembered when discussing his friendship with La Boetie: “If I am pressed to say why I loved him, I feel it can only be explained by replying: Because it was he; because it was me.” Thus the author of the Essays summed up the dazzling friendship which bound the two men from 1558 to 1563. La Boetie had, moreover, had a premonitory vision of this posthumous success, when he wrote to Montaigne in a long poem: "If fate wills it, posterity, be sure, will bear our two names on the list of famous friends."[19]

​

“Monday Mozart composes like Haydn, Tuesday Haydn composes like Mozart”. One might be tempted to see in this saying which was still running in Vienna in the middle of the 19th century the equivalent of Montaigne's sentence ... It is true that like Montaigne and La Boetie, Mozart and Haydn appear in the list of famous friends.
 
But, as Jean Massin[20] writes, the friendship between the two men is all the more beautiful as they are profoundly different both in character and in aesthetics.
 
Rather than a spiritual fatherhood, it is therefore a deep friendship coupled with a sincere admiration that defines the bonds between Mozart and Haydn. We will obviously add the Mason fraternity. It will be recalled that Mozart and Haydn were initiated into the same Lodge two months apart in December 1784 and February 1785.
 
Mozart has a sincere regard for Haydn and does not suffer him to be denigrated or belittled.
J. F. Rochlitz relates the anecdote of an exchange between Mozart and a composer who was very critical of a new work by Haydn: I would not have done it like this, nor like that ...
"Sir, - Mozart replied with extreme vivacity - even if we both melted together, it would still take time for a Haydn to emerge ... No one can do it all - play and touch, make people laugh and move - all this as well as Haydn ”.[21]
 
He never hides his admiring affection "He alone has the secret of making me smile, of touching me in the depths of my soul ...". This Mozart confidence shows how much the two men appreciate each other. Never did Mozart speak without the greatest esteem of this Master, although they lived in the same city so fond of musical novelties as much as of gossips that feed conversations and even though neither one nor the other were lacking excuses for reciprocal jealousy.
 
For his part, as soon as he met Mozart and heard his works, Haydn recognized the genius of Mozart.
 
In December 1787, a citizen of Prague asked Haydn to write an opera bouffe. Hire Mozart! He answered “Who can compare to the great Mozart? If I could only impress on the soul of every friend of music, and on high personages in particular, how inimitable are Mozart’s works, how profound, how musically intelligent, how extraordinarily sensitive! (for this is how I understand them, how I feel them) —then the nations would compete with each other not only to possess such a jewel within their frontiers… but also to keep and reward him. Life of great geniuses can be sad indeed, … It enrages me to think that this incomparable Mozart is not yet engaged in some imperial or royal court! Forgive me if I lose my temper. But I love this man so dearly.”
 
After Mozart death, Haydn wrote “I was a long time beside myself by the death of Mozart; I could not believe that Providence had called so soon an irreplaceable man. Above all, I regret that Mozart did not have before his death the opportunity to convince English people who still ignore him, of the truth of his talent that I preach to them daily. "

[15] Letter to his friend the composer C.F. Zelter.

[16] See S. Hinton in “Defining string quartet: Haydn” for Stanford School of Humanities and Sciences.

[17] See the opinion of J. Haydn on the evening of February 12, 1785, introduction op. cit.

[18] See on youtube the version by the Cambini-Paris Quartet live recorded for France musique.

[19] Wrote by Catherine Vincent for le Monde - article du 21 Juin 2005.

[20] See Jean & Brigitte Massin, - Mozart - Ed° CFL 1958 page 442 & s.
[21] By Johann Friedrich Rochlitz (1769-1842) German playwright, friend of Goethe, Schiller, also musicologist and art critic.