Why do we call a line of poetry a line in English when it is a vers in French (though we do use verse and verses to mean ‘poetry’)? What did Middle English poets call their lines of poetry? In their (at least) trilingual world, they had plenty of words for the basic units of poetry. A metrical line is a versus in Latin (and by extension versus means a set of verses, i.e. a poem). Continental and Insular French usage follows this: a vers is a metrical line. Vers signifies the core unit of poetry, marked out by repeating segments of metre and rhyme. Poetry was in verses whether it was copied as prose (with its units marked by metrical punctuation) or lineated as verse.
Words gain and change meanings by various processes, one of which is metonymic change, shifts of meaning which occur when the name of a related or contiguous thing is transferred over to a new referent. Lines of writing on the page take their name from the ruled lines a scribe makes on a manuscript. These graphic lines of writing, the rows of letters and words the scribe makes on the page, then by extension give their name to the units of text that are written on them. From the early fifteenth century, French uses ligne to refer to metrical lines as well as to graphic lines. Also by metonymy, vers in English and French soon refers to the form poetry itself (often as opposed to prose), that which is composed of lines of verse.
When and why did poets writing in English first refer to the metrical units of their own poems and what did they call their metrical lines? For a long time poets just got on with writing lines of poetry without stopping to refer to this obvious and defining aspect of their own form. Words like baston, and later staf, appear and disappear as short-lived words to name the metrical line when desired (though confusingly both might also mean ‘stanza’). Until the end of the fourteenth century, vers(e) mainly appears in Middle English when authors introduce citations of Latin (whether this be quotations from the Bible, especially the psalms, of proverbial wisdom, or for Latin poetry) and inscriptions on buildings. It doesn’t refer to the metrical units of English poetry, but then it probably doesn’t need to: it’s obvious that verse is verse.
Chaucer uses vers conventionally to refer to citations from Latin poetry. He uses line to refer to lines of writing, i.e. graphic lines in letters and documents. But he also calls his own metrical lines vers (the same form for singular and plural), as one might exactly expect given his francophone training and reading. When Chaucer refers to the ‘next vers’ (Troilus and Criseyde) or ‘next this vers’ (Parliament of Fowls), he means not the next stanza (as an unwary modern reader might think) but the next line. In the House of Fame, he concedes that ‘som vers’ might ‘fayle in a sillable’: a metrical line might be a syllable short. The Man in Black in the Book of the Duchess sorrowfully utters his complaint ‘of rym ten vers or twelve’, and what Chaucer gives us has eleven lines.
Chaucer thus starts the fashion for metaliterary deixis. This is a specialised kind of discourse deixis, using technical terms of poetics to self-reflexively describe poetry’s own construction and/or to locate the reader in relation to elements of the construction. The particular choice of technical term tells us something about the sort of poetry this is claiming to be. Chaucer counts syllables, just as a French poet does, and not surprisingly he thinks of his lines as vers. He makes, I think, a different point when he calls Troilus and Criseyde ‘Thise woful vers’ (echoing Boethius’s description of the ‘heartbreaking verse’ and ‘woeful songs’ that he composes in his miserable imprisonment in The Consolation of Philosophy). Rather like the end of Troilus, where the poem is told to subject itself to classical poetry rather than to rival English poems, this is a small but meaningful humble-brag at the poem’s start. Calling Troilus ‘vers’ associates it with classical poetry rather than English narrative forms.
For some fifteenth-century poets, vers and line are interchangeable. James I of Scotland in the Kingis Quair (?1424) names his own stanza-units as ‘thir versis sevin’ (referring to a one-stanza song) and as ‘my buk in lynis sevin’. James articulates his lines so as to count them, to flag up his choice of Chaucerian stanza form. Just as French by 1420 uses ligne and vers, so James uses both terms, just as he uses other French terms like ‘copill’ for stanza.
John Lydgate, a Benedictine monk with a university training at Oxford, thinks a little differently. He reserves the word verse in his English poetry for references to Latin poetry (as far as I can tell). When referring to his own metrical units in the Troy Book (probably the first poem of his career, written from 1412–20), he uses the word line. He says that he needs Chaucer’s help to find some good words to put in ‘the crokid lynys rude / Which I do write’. Near the end of this long narrative, he tells us that he must ‘spende a fewe lines blake’ translating the last chapter of his source. This sounds like humility: calling his work mere graphic lines on a page, merely ink.
Humility topoi, though, are often fifteenth-century humble-brags. Just before Lydgate reaches the last chapter of his source, the Historia Destructionis Troiae, he tells us that he has nearly reached the ‘boundis set of my labour’, the edges of the plot of land set for him to work. He can hardly summon the strength to write: ‘the bestes and oxes of my plow’ are weary, faint and weak to keep climbing up the hill all day long. Soon, however, he will fix ‘a stake’ ‘at the londes ende / Of Troye boke’ and be done with his long translation. He pictures his work as a ploughed field, organized as it was in shared medieval open-field farming where individual plough-strips were marked out by stakes.

This agricultural imagery claims Lydgate’s lines as metrical verses without ever having to say so. In his encyclopaedia, the Etymologies, Isidore of Seville explains that versus means both a furrow and a metrical line because ancient poets would supposedly write from left to right and then turn the pen (like a plough turning at the end of a field) to the line below from right to left. So Lydgate’s ploughing is versification, and we are being invited to admire his long technical labour.
Something similar is going on with the idea that Lydgate’s lines are ‘crokid lynys rude’. You might think that a line of verse written on a straight ruled line can’t be crooked in any literal sense (unless the scribe was really making a mess of things). But crokid in Middle English means ‘lame’ as well as wonky. This lameness is another gesture at versification. For something to limp it must have feet (English and French poems, admittedly, don’t have feet in the classical sense, but English and French poets used ‘feet’ to mean syllable). Lydgate feigns that his verse limps, that his lines are crooked, but this humble-brag nudges the reader to remember the metricality of his verse that runs on its feet unstopped for 30,000 lines.
Eventually, verse meaning ‘metrical line’ becomes much less popular than ‘line’ for referring to a metrical unit, though we still talk about verses and verse. I think that this is because verse in the late medieval period increasingly refers to part of a song, usually the non-repeating part as opposed to the burden or chorus. Verse names a part of something which is sung, whether that is a liturgical versicle, a psalm, or a secular song. Most of the early citations for verse as ‘stanza’ appear in the context of music or singing. It wasn’t until the sixteenth century that this meaning migrated metonymically from song lyrics to lyrics-as-poems and then as a conventional way of describing stanzas. Verse in English then mostly refers to stanzas (or to poetry generally), with line having taken over as the standard term for a metrical line.