Chaucer's first major work, The Book of the Duchess, was an elegy for Blanche of Lancaster (who died in 1369). It is possible that this work was commissioned by her husband John of Gaunt, as he granted Chaucer a £10 annuity on 13 June 1374. This would seem to place the writing of The Book of the Duchess between the years 1369 and 1374. Two other early works by Chaucer were Anelida and Arcite and The House of Fame. Chaucer wrote many of his major works in a prolific period when he held the job of customs comptroller for London (1374 to 1386). His Parlement of Foules, The Legend of Good Women and Troilus and Criseyde all date from this time. Also it is believed that he started work on The Canterbury Tales in the early 1380s. Chaucer is best known as the writer of The Canterbury Tales, which is a collection of stories told by fictional pilgrims on the road to the cathedral at Canterbury; these tales would help to shape English literature.
The Canterbury Tales contrasts with other literature of the period in the naturalism of its narrative, the variety of stories the pilgrims tell and the varied characters who are engaged in the pilgrimage. Many of the stories narrated by the pilgrims seem to fit their individual characters and social standing, although some of the stories seem ill-fitting to their narrators, perhaps as a result of the incomplete state of the work. Chaucer drew on real life for his cast of pilgrims: the innkeeper shares the name of a contemporary keeper of an inn in Southwark, and real-life identities for the Wife of Bath, the Merchant, the Man of Law and the Student have been suggested. The many jobs that Chaucer held in medieval society—page, soldier, messenger, valet, bureaucrat, foreman and administrator—probably exposed him to many of the types of people he depicted in the Tales. He was able to shape their speech and satirise their manners in what was to become popular literature among people of the same types.
Chaucer's works are sometimes grouped into first a French period, then an Italian period and finally an English period, with Chaucer being influenced by those countries' literatures in turn. Certainly Troilus and Criseyde is a middle period work with its reliance on the forms of Italian poetry, little known in England at the time, but to which Chaucer was probably exposed during his frequent trips abroad on court business. In addition, its use of a classical subject and its elaborate, courtly language sets it apart as one of his most complete and well-formed works. In Troilus and Criseyde Chaucer draws heavily on his source, Boccaccio, and on the late Latin philosopher Boethius. However, it is The Canterbury Tales, wherein he focuses on English subjects, with bawdy jokes and respected figures often being undercut with humour, that has cemented his reputation.
Chaucer also translated such important works as Boethius' Consolation of Philosophy and The Romance of the Rose by Guillaume de Lorris (extended by Jean de Meun). However, while many scholars maintain that Chaucer did indeed translate part of the text of Roman de la Rose as The Romaunt of the Rose, others claim that this has been effectively disproved. Many of his other works were very loose translations of, or simply based on, works from continental Europe. It is in this role that Chaucer receives some of his earliest critical praise. Eustache Deschamps wrote a ballade on the great translator and called himself a "nettle in Chaucer's garden of poetry". In 1385 Thomas Usk made glowing mention of Chaucer, and John Gower, Chaucer's main poetic rival of the time, also lauded him. This reference was later edited out of Gower's Confessio Amantis and it has been suggested by some that this was because of ill feeling between them, but it is likely due simply to stylistic concerns.
One other significant work of Chaucer's is his Treatise on the Astrolabe, possibly for his own son, that describes the form and use of that instrument in detail and is sometimes cited as the first example of technical writing in the English language. Although much of the text may have come from other sources, the treatise indicates that Chaucer was versed in science in addition to his literary talents. Another scientific work discovered in 1952, Equatorie of the Planetis, has similar language and handwriting compared to some considered to be Chaucer's and it continues many of the ideas from the Astrolabe. Furthermore, it contains an example of early European encryption. The attribution of this work to Chaucer is still uncertain.