Elsewhere it's commented, "Chaucer's Canterbury Tales are in Middle English. Take it back another 600 years and you have Old English."

Do we have the dates right? I didn't think Middle English lasted so long as 600 years.

Come to think of it, how do we draw the boundary between Old and Middle English, and the boundary between Middle and Modern English?

Come to think of it, how do we draw the boundary between Old and Middle English, and the boundary between Middle and Modern English?

Well the usual dates given are Old English ca.450-1100 CE, Middle English 1100-1500, and Present Day English 1500-.

The transition between OE and ME took place during a politically unstable period when the English monarchy was being transitioned from the English to the Normans. During the 16 century, the English vowel shift took place: where many vowels changed pronounciation from the arlier (similar to the Italian vowel system) to the later (similar to the English vowel system). Unfortunately, printing was also being developed during this period and the orthography of English choose some mighty strange ways to spell stuff.

One highly visible change from OE to ME is the phonetic reduction that changed the grammar: the complicated OE noun endings had been reduced to -e -en -es. This started before the Conquest, and was complete by 1100. Another characteristic sign is past participles in i-/y- instead of ge-.

In vocabulary the major change is the French influx, which took off from 1150. Before this only technical terms of religion and government were borrowed; afterwards masses of everyday terms.

A text from the South of England from 1200 is readable without study of OE. Modern Standard English is roughly a phonetic rendition of a southern dialect of 1400, so the spellings of 1200 are close enough to make out.

As to the boundary between Middle-English, somewhere between 1400 and 1500: Caxton, in a famous 1490 passage, laments how difficult it is to render a work in "English" when the language is changing so fast and differs so greatly from place to place. He tells an amusing incident of a merchant who, wanting eggs, couldn't explain himself to a woman who used a different dialect word.

The content tells you the situation of the times, but you'll also be struck by how easy it is to understand, more than five centuries later. To me, it's just as easy to read as the Scots site that neveu gave us. I'm reproducing it verbatim, except that as the letters v and u were used interchangeably, I've substituted to conform with modern usage. A translation appears below it, in white type, but I seriously doubt you'll need it

Caxton: And also my lorde abbot of Westmynster ded do shewe to me late certayn evydences wryton in olde Englysshe for to reduce it into our Englysshe now usid, And certaynly it was wreton in suche wyse that it was more lyke to Dutche than Englysshe; I coude not reduce ne brynge it to be understonden. And certaynly our langage now used varyeth ferre from that whiche was used and spoken whan I was borne. For we Englysshe men ben borne under the domynacyon of the mone [moon], whiche is never stedfaste but ever waverynge, wexynge one season, and waneth and dyscreaseth another season.
. . . And that comyn Englysshe that is spoken in one shyre varyeth from a nother. In so moche that in my dayes happened that certayn marchauntes were in a shippe in Tamyse, for to have sayled over the see into Zelande, and for lacke of wynde thei taryed atte Forlond, and wente to lande for to refreshe them; And one of theym named Sheffelde, a mercer, cam in-to an hows and axed for mete; and specyally he axyd after eggys; And the goode wyf answerde, that she coude not speke no Frenshe. And the marchaunt was angry, for he also coude speke no Frenshe, but wolde have hadde egges, and she understode hym not. And thenne at laste a nother sayd that he wolde have eyren; then the good wyf sayd that she understod hym wel.
. . . Loo, what sholde a man in thyse dayes now wryte, egges or eyren. Certaynly it is harde to playse everyman by cause of dyversite and chaunge of langage.

Caxton, "transated" (in white font; paint over to view): And also my lord abbot of Westminster did do show to me late certain evidences written in old English for to reduce it into our English now used. And certainly it was written in such wise that it was more like to German than English; I could not reduce nor bring it to be understood. And certainly our language now used varies far from that which was used and spoken when I was born.. For we English men are borne under the domination of the moon, which is never steadfast but ever wavering, waxing one season, and wanes and decreases another season.
. . . And that common English that is spoken in one shire varies from another. In so much that in my days happened that certain merchants were in a ship in Thames, for to have sailed over the sea into Zelande, and for lack of wind they tarried at Forlond, and went to land for to refresh them; And one of them named Sheffield, a mercer, came into a house and asked for meat; and specially he asked after eggys; And the good wife answered, that she could not speak no French. And the merchant was angry, for he also could speak no French, but would have had egges, and she understood him not. And then at last another said that he would have eyren; then the good wife said that she understood him well.
. . . Loo, what should a man in these days now write, egges or eyren? Certainly it is hard to please everyman by cause of diversity and change of language.