Rhythm and rubato
In a conversation with his former teacher Ernest Guiraud, Debussy spoke about his ideas for a new kind of music. He explained his idea of a flowing rhythm:
Rhythms cannot be contained within bars. It is nonsense to speak of 'simple' and 'composed' time. There should be an interminable flow of them both without seeking to bury the rhythmic patterns. [Lockspeiser (1962) p. 206]
This idea of a flowing rhythm was also hinted at in the last chapter: "It is necessary to abandon yourself completely, and let the music do as it will with you - to be a vessel through which it passes." [Nichols p. 167]
This would mean that sudden changes in rubato, something that was common for pianists in the romantic era, should be avoided. At the same time it is important that the music doesn't become stiff and mechanic. It should flow freely and naturally. Schmitz, in his book, says just this:
In Debussy interpretation an overabundance of rubato, of arbitrary fluctuations in tempo, has long been current; yet performance metronomically throughout, in his works, is just as unthinkable. [Schmitz p. 38]
Dumesnil tells us that Debussy insisted that the triplets in Hommage à Rameau be completely rhtythmical [Nichols p. 158]. Also in Clair de lune it was important that the triplets were strictly in tempo, "but within a general flexibility." [Nichols p. 159].
In romantic music triplets, when they act as contrast to the regular rhtythm, are often played with a kind of rubato where the first tone is prolonged with an agogic accent [Philip; Hudson]. This is not what Debussy wants, but they should still be "flexible". According to Dumesnil, Debussy's view was that rubato should rather be done on the phrase as a whole. [Nichols p. 159].
In the two first bars of Reflets dans l'eau, Dumesnil describes such a rubato where the whole phrase is flexible:
“Tempo rubato” applies to the delivery of the two bars as a whole, not to any individual beats. One can start slowly, get slightly faster in the middle, “easing up” again towards the end. [Dumesnil p. 15]
Dumesnil also tells about his experience of playing Poissons d'or for Debussy, and there is some frustration in the text over not quite understanding Debussy's idea of rhythm and flow in this piece:
With 'Poissons d'or' it was indeed difficult to satisfy Debussy. 'Jouez plus librement,' he would repeat. I thought I did play with great freedom, but it was not enough. [...] Toward the middle he spoke again: 'Plus gracieux, plus élégant.' But when I complied, he said: 'Jouez plus simplement.' I came to the conclusion that the interpretation of Ricardo Viñes, to whom 'Poissons d'or' is dedicated, had become inseparable from his own conception; so I took it as a model and susequently won approval.[Nichols p. 160]
From this account one understands that Debussy was after a considerable amount of freedom and flexibility in the piece, and that it should by no means be played mechanically. Still it should have some simplicity. It is thus clear that Debussy's rubato is different from the typical romantic rubato, where impulsiveness and sudden tempo changes are common, and where one can stretch single notes with an agogic accent. The phrases should on the contrary have a natural flexibility.
Terms like "flexible" and "natural" are rather unspecific. Debussy would seem to prefer not to describe rubato, rhythm and tempo in great detail. He said something about this in a letter to Manuel de Falla. De Falla was to perform a piano version of Danses (originally for harp and orchestra), in Madrid, and had evidently written to Debussy for advice on performance of the work. Debussy writes back 13th of january 1907:
What you ask is rather hard to give a definite answer to! It's not possible to write down the exact form of a rhythm, any more than it is to explain the different effects of a single phrase! The best thing, I think, is to be guided by how you feel...The colour of the two dances seems to me to be clearly defined. There's something to be got out of the passage between the 'gravity' of the first one and the 'grace' of the second; for a musician such as yourself that will not be difficult, and I am quite happy to leave the performance to your good taste.[Lesure & Nichols p. 176]
Edgar Varèse, who knew Debussy, also asked Debussy for advice on performance, and requested metronom indications for Pelléas. 12th of juli 1910 Debussy answered:
Send me the Pelléas score and, although I have no confidence in metronome markings, I'll do what you ask.[Lesure & Nichols p. 222]
There do exist some metronom markings in Debussy scores. However, in a letter to his publisher Durand 9th of october 1915, we can read about how sceptical he was to fixate the tempo like this. His opinion was that the performer ought to "hear" the piece himself and "feel" the natural tempo:
You know what I think about metronome marks: they're right for a single bar, like 'roses, with a morning life'. Only there are 'those' who don't hear music and who take these marks as authority to hear it still less! But do what you please.[Lesure & Nichols p. 305]
There are hardly any traces in the written sources of what Debussy's opinion on dotted rhythm was. A dotted rhythm was often performed overdotted in the beginning of the 20th century [Philip]. It would seem that Debussy had a more modern view in that the rhythms should be performed exactly, since he stressed that triplets should be played "strictly in tempo". However, according to Howat, the french pianist Marcel Ciampi (1891-?) claimed that Debussy wanted the ostinato rhythm of the habanera in La puerta del vino and La soirée dans Grenade overpunktert overdotted. [Howat (1995) p. 13]