Geoffrey Chaucer (c. 1343 – October 25, 1400) was an English author, poet, philosopher, bureaucrat (courtier), and diplomat. He is often referred to as the Father of English Literature. Although he wrote many works he is best remembered for his unfinished frame narrative The Canterbury Tales. He is sometimes credited with being the first author to demonstrate the artistic legitimacy of the vernacular English language, rather than French or Latin.

Chaucer wrote in continental accentual-syllabic metre, a style which had developed since around the twelfth century as an alternative to the alliterative Anglo-Saxon metre. Chaucer is known for metrical innovation, inventing the rhyme royal, and he was one of the first English poets to use the five-stress line, the iambic pentameter, in his work, with only a few anonymous short works using it before him. And the arrangement of these five-stress line into rhyming couplets was first seen in his The Legend of Good Women, was used in much of his later work and became one of the standard poetic forms in English. His early influence as a satirist is also important, with the common humorous device, the funny accent of a regional dialect, apparently making its first appearance in The Reeve's Tale.

The poetry of Chaucer, along with other writers of the era, is credited with helping to standardise the London Dialect of the Middle English language; a combination of Kentish and Midlands dialect. This is probably overstated: the influence of the court, chancery and bureaucracy — of which Chaucer was a part — remains a more probable influence on the development of Standard English. Modern English is somewhat distanced from the language of Chaucer's poems owing to the effect of the Great Vowel Shift some time after his death. This change in the pronunciation of English, still not fully understood, makes the reading of Chaucer difficult for the modern audience, though it is thought by some that the modern Scottish accent is closely related to the sound of Middle English.

The status of the final -e in Chaucer's verse is uncertain: it seems likely that during the period of Chaucer's writing the final -e was dropping out of colloquial English and that its use was somewhat irregular. Chaucer's versification suggests that the final -e is sometimes to be vocalised, and sometimes to be silent; however, this remains a point on which there is disagreement. When it is vocalised, most scholars pronounce it as a schwa  //. Apart from the irregular spelling, much of the vocabulary is recognisable to the modern reader.

Chaucer is also recorded in the Oxford English Dictionary as the first author to use many common English words in his writings. These words were probably frequently used in the language at the time but Chaucer, with his ear for common speech, is the earliest manuscript source. Acceptable, alkali, altercation, amble, angrily, annex, annoyance, approaching, arbitration, armless, army, arrogant, arsenic, arc, artillery and aspect are just some of those from the first letter of the alphabet.