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The Yénonótië was a great work by the Loremaster Quennar in which he explained the chronology of the Valar. It included discussions on the lengths of Valian Years, the ages of the Valar, and the Years of the Sun of the First Age. Some of the material in the Yénonótië was later quoted by Pengolodh in his enlargement of the Annals of Aman.
Etymology
Yénonótië means "Reckoning of Years" in Quenya (from the words yén 'long year' and onótië, 'reckoning').