Synesthesia is a literary term used to describe a deliberate mixing of the senses, for example when we talk about loud colours, sharp scents or sweet music. This is often used in a more complex manner in poetry and other literature to striking poetic effect.
“Stephen [...] heard warm running sunlight”
– James Joyce, Ulysses<
The technique was used by poets of the Romantic Period, such as Shelley and Keats, and also by French Symbolists such as Baudelaire and Rimbaud.
In one of my favourite novels, Against Nature (À Rebours), author J.K. Huysmans uses synesthesia as he describes how different liqueurs taste like the sounds of different instruments for Des Esseintes, the novel’s main protagonist:
“Des Esseintes would drink a drop here, another there, playing internal symphonies to himself, and providing his palate with sensations analogous to those which music dispenses to the ear.
Indeed, each and every liqueur, in his opinion, corresponded in taste with the sound of a particular instrument. Dry curaçao, for instance, was like the clarinet with its piercing velvety note; kümmel like the oboe with its sonorous, nasal timbre; crème de menthe and anisette like the flute, at once sweet and tart, soft and shrill. Then to complete the orchestra there was kirsch, blowing a wild trumpet blast; gin and whisky raising the roof of the mouth with the blare of their coronets and trombones; marc-brandy matching the tubas with its deafening din; while peals of thunder came from the cymbal and bass drum, which arak and mastic were banging and beating with all their might.”
This then continues for another full page as he describes the various delights of an old brandy (violin), rum (viola), Benedictine (which is in a minor key) and green Chartreuse (a major key). He re-creates existing melodies or
“At other times he would compose melodies of his own, executing pastorals with the sweet blackcurrant liqueur that filled his throat with the warbling song of the nightingale; or with the delicious cacaochouva that hummed sugary bergerets [...]“
I love this type of metaphor!