There’s a certain sub-class of English nouns that has interested me for a while. They’re formed by the suffixation of /-th/ to an adjective, yielding a noun. What’s really peculiar is the alternation of vowel sounds between the two words; in the nominal form, the vowel is one thing, and in the adjectival, it’s another. Here are two common examples:

long - length
strong - strength
broad - breadth

In each case, the stem vowel changes the place where it’s pronounced in the mouth. This is called “I-mutation.” Basically, in the adjectival form — which is derived from the noun — the vowel scoots closer to the sound /i/ (”ee” as in feet) In this case, the shift is from /ɑ/→/ε/.*

The chart below visually demonstrates the process. It’s essentially a location map of where your tongue is in your mouth during the execution of vowels. For the /ɑ/ in long, strong and broad your tongue is low and retracted, and for /i/ of feet its pushed forward and up.

Figure 1: /ɑ/→/ε/

As you can see, the sound /ɑ/ is pulled upward and forward, and winds up at an intermediate between /ɑ/ and /i/, namely /ε/.

Why would the vowel undergo this transformation in the first place? Well historically, the /-th/ suffix was more like /-ith/, so it actually contained the sound /i/ (”ee”). In anticipation of the vowel in the suffix, speakers would move the stem vowel incrementally closer to /i/, and they stopped off somewhere in between. It has been said that this is a form of laziness, in which speakers try to expend as little effort as possible to pronounce the word, but I think it’s more likely a constraint on articulation: the tongue is physical object, and must actually move from one position to another. Over time, this transition becomes audible, and begins to color the vowels. Eventually, the “slide” between sounds becomes part of the accent of that language and becomes established (think of a Spanish speaker trying to say home, they have much trouble mastering the slide from “oh” to “oo” packed into that vowel). But I digress.

For the other words in this class, the process of I-Mutation has been obscured by other processes. For example, in the following words, the adjective was derived from the noun and was preserved. Later, the Great Vowel Shift changed the sound of the noun, but left the adjective intact:

fūl → filth

Figure 2: /u:/→/i/

Later, fūl (pronounced “fool”) became foul by means of the Great Vowel shift: /u:/→/au/

hāl → health /ɑ/→/ε/ (See Fig. 1)

Here, the Great Vowel Shift covered I-Mutation’s tracks by changing hāl (pronounced “hall”) to whole and a less commonly used word hale. The likely explanation for the two results is that there were two competing pronunciations of hāl, and each went a different way with the Great Vowel Shift (and it probably has to do with the influence of /l/ on the preceding vowel … don’t ask).

In other instances, it’s more complicated. The Old English word slaw had the same vowel of modern (cole)slaw, but the /w/ was actually pronounced (like the interjection “ow!” but with the /ɑ/ of father instead of the /æ/ of cat). I-Mutation pulled the /ɑ/ forward:

Figure 3: /ɑ/→/æ/

… and we were left with slæwth, which stuck around for quite a while in Old English. However, in Middle English, slaw changed roughly to the modern pronunciation slow, and speakers – conscious of the relationship between slow and slæwth – futzed the vowels. Essentially, they performed the equivalent of changing length to longth. The important thing is that the /th/ stuck around.

One final example is young - youth, although the story of where the /ng/ went is a bit convoluted.

All of the derivations we’ve examined thus far were formed in Old English and were fossilized. At some point the /-ith/ suffix ceased being a preferred way of forming nouns, and it was abandoned in favor of suffixes like /-ness/, but the nouns that had already been formed were preserved. Linguists say suffixes such as /-ith/ are no longer “productive,” because they’re recognizable in a few places, but they aren’t actively used in the generation of new words.

However, since the process of adding /-ith/ and I-mutating the stem vowel fell out of use, some words have nevertheless been formed to superficially resemble the fossilized forms through the process of analogy. For example, wealth was formed from well under analogy to health. And tellingly, there was no application of I-Mutation: well and wealth have the same vowel.

Meanwhile, other words were analogized that DO exhibit some change in the stem vowel, but not in the manner we know to be consistent with I-Mutation. Depth was formed under analogy with breadth, but I-Mutation isn’t responsible for turning deep to depth. Similarly, analogy to breadth is also the provenance of width (from wide).

Most interesting is the emergent tendency to analogize to breadth for words relating to dimensions. It’s as if speakers wanted wide and deep to behave like long and broad, so that they could all be one neat set. This “ironing out” the kinks in the language is still happening today (although there’s some out there who would fight it). The noun height is traditionally formed from the adjective high, but you can often hear people saying heighth! It’s not hard to imagine a day when heighth has become accepted, and all the words for dimensions end in /th/.

*Technically, it was probably /ɔ/→/œ/ which was subsequently unrounded to /ε/. That’s why we have “o” in spelling today, but you get the idea.

From my perspective as a linguist, Spanish is a rather unremarkable language. It has a relatively normal sound inventory (no ejectives, clicks, or voiced aspirates) it has a standard verbal conjugation system reflecting person and number, and its syntax is, for the most part, vanilla Subject-Verb-Object — like English.

However, there was always one thing about this language that bothered me: the past tense of its verb ‘to be’ is the same as the past tense of its verb ‘to go,’ ser and ir respectively. Take a look at the following tables for comparison:

Exactly the same! Freaky, I know.

I always wondered how a language could get by without the ability to distinguish ‘went’ from ‘was’ — it seems like such an indispensable distinction. Other related languages, like French, maintain two separate words for these concepts, as did Latin. So this got me thinking: what happened to Spanish along the way that allowed these verbal paradigms to collapse into one?

As always, the past is the key to the present. In Latin, the word ‘to go’ was the tiny little word īre. The macron over the i means that it was pronounced twice, so another way to write it could be iire. However, given that the morphological marker for the infinitive in Latin was [vowel]re, the stem of this verb was even smaller; it was really just i. Not a very substantial morpheme, and it tended to get bullied around a bit, sometimes showing up as e. Take a look at the present tense:

īre ‘to go’ (present)

The blue parts are conjugational endings that are added to every verb. For our purposes, they don’t even count as part of the word, so you can really get the feel that this word was a feeble little guy.

Most of the romance languages were of the same opinion, so one of the first things they did was dispense of īre, usually replacing it with a version of the word vadīre, which meant ‘wander.’* This is where the French get their present forms vais, vas, va and vont, and where Spanish gets voy, vas, va, vamos, vais, and van (although both languages retain some forms that use the base ir, and we will see why in a moment).

Having developed a propensity for suppletion, which is the technical term for replacing part of a verb’s paradigm with forms of another verb, īre was ready to lose its past tense. Fortunately, there was already a good candidate for replacement, and it came from an alternative past tense expression using the word ‘be’ followed by an accusative. We have the same construction in English: ‘We’ve been to France.’ Over time, this expression became the predominant means of expressing ‘go’ in the past tense, and so ser slowly replaced the original past tense of īre.

When no reasonable alternative to īre was available, the languages were stuck with it. This is why today Spanish still forms its futures, imperfects, and conditionals on a descendant of īre — iré, iras; iba, ibas; and iría, irías etc. French uses the stem ir- as a base for futures and conditionals — j’irai, j’irais — but has performed even further suppletion, replacing the imperfect, preterite, and some present forms with the base all-, borrowed from neighboring Germanic languages.

So, as you can see, what appears today to be totally nonsensical irregularity has a completely sensical, regular origin. Understanding the mechanisms that are responsible for creating the irregularity in the conjugation of ‘to go’ in Spanish sheds light on the irregularity of verbs like aller in French, whose forms vary unpredictably from je vais, to j’allais, to j’irai. Ultimately, these processes even illuminate the origins of highly irregular English paradigms like go/went and be/is/was.

Historical linguistics is cool!

*Interestingly enough, English did a similar thing. Our past tense went is taken from the word wend, which means something close to ‘wander’. It supplanted the original past tense of go.