Twenty Lessons In Harmony by Olivier Messiaen is a book in which the author describes his perceptions of colours in music and the importance of the fusion of sound and sight. Often depicting chords as precise colours and shades he explains how he perceived harmony through a visual prism. This description is completed by these Twenty Lessons for SATB to help controlling the harmonies. Twenty Lessons In Harmony has shown itself very helpful for a better understanding and interpretation of Messiaen’s music. Olivier Messiaen (1908-1992) was a Frenchorganist and composer passionate about Ornithology and one of the most important composers of his century. Inspired by Japanese music he had a very special way of composing and his work can be identified by its complexity its diatonic aspect its harmony with limited transposition its colour and its additive rhythms. He composed many works related to ornithology and birdsong including the 'Bird Catalogue' in 7 volumes and the 'Treatise on rhythm colour and ornithology' in 7 volumes.