The Book of the Duchess, also known as The Deth of Blaunche, is the earliest of Chaucer's major poems, preceded only by his short poem, "An ABC", and possibly by his translation of The Romaunt of the Rose. Based on the themes and title of the poem, most sources put the date of composition after 12 September, 1368 (when Blanche of Lancaster died) and before 1372, with many recent studies privileging a date as early as the end of 1368.
Overwhelming (if disputed) evidence suggests that Chaucer wrote the poem to commemorate the death of Blanche of Lancaster, wife of John of Gaunt. The evidence includes handwritten notes from Elizabethan antiquary John Stowe indicating that the poem was written at John of Gaunt's request. There are repeated instances of the word "White", which is almost certainly a play on "Blanche". In addition, at the end of the poem there are references to a 'long castel', suggesting the house of Lancaster (line 1318) and a 'ryche hil' as John of Gaunt was earl of Richmond (mond=hill) (line 1319) and the narrator swears by St John, which is the name of John of Gaunt's saint.