One of the things historical linguists do is reconstruct relative chronologies: statements about whether one change in a language occurred before another change in the language. For example, in the history of English there was a change which raised the Middle English (ME) mid back vowel /oː/, so that it became high /uː/: boot, pronounced /boːt/ in Middle English, is now pronounced /buːt/. There was also a change which caused ME /oː/ to be reflected as short /ʊ/ before /k/ (among other consonants), so that book is now pronounced as /bʊk/. There are two possible relative chronologies of these changes: either the first happens before the second, or the second happens before the first. Now, because English has been well-recorded in writing for centuries, because these written records of the language often contain phonetic spellings, and because they also sometimes communicate observations about the language’s phonetics, we can date these changes quite precisely. The first probably began in the thirteenth century and continued through the fourteenth, while the second took place in the seventeenth century (Minkova 2015: 253-4, 272). In this particular case, then, no linguistic reasoning is needed to infer the relative chronology. But much of if not most of the time in historical linguistics, we are not so lucky, and are dealing with the history of languages for which written records in the desired time period are much less extensive, or completely nonexistent. Relative chronologies can still be inferred under these circumstances; however, it is a methodologically trickier business. In this post, I want to point out some complications associated with inferring relative chronologies under these circumstances which I’m not sure historical linguists are always aware of.
Let’s begin by thinking again about the English example I gave above. If English was an unwritten language, could we still infer that the /oː/ > /uː/ change happened before the /oː/ > /ʊ/ change? (I’m stating these changes as correspondences between Middle English and Modern English sounds—obviously if /oː/ > /uː/ happened first then the second change would operate on /uː/ rather than /oː/.) A first answer might go something along these lines: if the /oː/ > /uː/ change in quality happens first, then the second change is /uː/ > /ʊ/, so it’s one of quantity only (long to short). On the other hand, if /oː/ > /ʊ/ happens first we have a shift of both quantity and quality at the same time, followed by a second shift of quality. The first scenario is simpler, and therefore more likely.
Admittedly, it’s only somewhat more likely than the other scenario. It’s not absolutely proven to be the correct one. Of course we never have truly absolute proofs of anything, but I think there’s a good order of magnitude or so of difference between the likelihood of /oː/ > /uː/ happening first, if we ignore the evidence of the written records and accept this argument, and the likelihood of /oː/ > /uː/ happening first once we consider the evidence of the written records.
But in fact we can’t even say it’s more likely, because the argument is flawed! The /uː/ > /ʊ/ would involve some quality adjustment, because /ʊ/ is a little lower and more central than /uː/. Now, in modern European languages, at least, it is very common for minor quality differences to exist between long and short vowels, and for lengthening and shortening changes to involve the expected minor shifts in quality as well (if you like, you can think of persistent rules existing along the lines of /u/ > /ʊ/ and /ʊː/ > /uː/, which are automatically applied after any lengthening or shortening rules to “adjust” their outputs). We might therefore say that this isn’t really a substantive quality shift; it’s just a minor adjustment concomitant with the quality shift. But sometimes, these quality adjustments following lengthening and shortening changes go in the opposite direction than might be expected based on etymology. For example, when /ʊ/ was affected by open syllable lengthening in Middle English, it became /oː/, not /uː/: OE wudu > ME wood /woːd/. This is not unexpected, because the quality difference between /uː/ and /ʊ/ is (or, more accurately, can be) such that /ʊ/ is about as close in quality to /oː/ as it is to /uː/. Given that /ʊ/ could lengthen into /oː/ in Middle English, it is hardly unbelievable that /oː/ could shorten into /ʊ/ as well.
I’m not trying to say that one should go the other way here, and conclude that /oː/ > /ʊ/ happened first. I’m just trying to argue that without the evidence of the written records, no relative chronological inference can be made here—not even an insecure-but-best-guess kind of relative chronological inference. To me this is surprising and somewhat disturbing, because when I first started thinking about it I was convinced that there were good intrinsic linguistic reasons for taking the /oː/ > /uː/-first scenario as the correct one. And this is something that happens with a lot of relative chronologies, once I start thinking about them properly.
Let’s now go to an example where there really is no written evidence to help us, and where my questioning of the general relative-chronological assumption might have real force. In Greek, the following two very well-known generalizations about the reflexes of Proto-Indo-European (PIE) forms can be made:
- The PIE voiced aspirated stops are reflected in Greek as voiceless aspirated stops in the general environment: PIE *bʰéroh2 ‘I bear’ > Greek φέρω, PIE *dʰéh₁tis ‘act of putting’ > Greek θέσις ‘placement’, PIE *ǵʰáns ‘goose’ > Greek χήν.
- However, in the specific environment before another PIE voiced aspirated stop in the onset of the immediately succeeding syllable, they are reflected as voiceless unaspirated stops: PIE *bʰeydʰoh2 ‘I trust’ > Greek πείθω ‘I convince’, PIE *dʰédʰeh1mi ‘I put’ > Greek τίθημι. This is known as Grassman’s Law. PIE *s (which usually became /h/ elsewhere) is elided in the same environment: PIE *segʰoh2 ‘I hold’ > Greek ἔχω ‘I have’ (note the smooth breathing diacritic).
On the face of it, the fact that Grassman’s Law produces voiceless unaspirated stops rather than voiced ones seems to indicate that it came into effect only after the sound change that devoiced the PIE voiced aspirated stops. For otherwise, the deaspiration of these voiced aspirated stops due to Grassman’s Law would have produced voiced unaspirated stops at first, and voiced unaspirated stops inherited from PIE, as in PIE *déḱm̥ ‘ten’ > Greek δέκα, were not devoiced.
However, if we think more closely about the phonetics of the segments involved, this is not quite as obvious. The PIE voiced aspirated stops could surely be more accurately described as breathy-voiced stops, like their presumed unaltered reflexes in modern Indo-Aryan languages. Breathy voice is essentially a kind of voice which is closer to voicelessness than voice normally is: the glottis is more open (or less tightly closed, or open at one part and not at another part) than it is when a modally voiced sound is articulated. Therefore it does not seem out of the question for breathy-voiced stops to deaspirate to voiceless stops if they are going to be deaspirated, in a similar manner as ME /ʊ/ becoming /oː/ when it lengthens. Granted, I don’t know of any attested parallels for such a shift. And in Sanskrit, in which a version of Grassman’s Law also applies, breathy-voiced stops certainly deaspirate to voiced stops: PIE *dʰédʰeh1mi ‘I put’ > Sanskrit dádhāmi. So the Grassman’s Law in Greek certainly has to be different in nature (and probably an entirely separate innovation) from the Grassman’s Law in Sanskrit.
Another example of a commonly-accepted relative chronology which I think is highly questionable is the idea that Grimm’s Law comes into effect in Proto-Germanic before Verner’s Law does. To be honest, I’m not really sure what the rationale is for thinking this in the first place. Ringe (2006: 93) simply asserts that “Verner’s Law must have followed Grimm’s Law, since it operated on the outputs of Grimm’s Law”. This is unilluminating: certainly Verner’s Law only operates on voiceless fricatives in Ringe’s formulation of it, but Ringe does not justify his formulation of Verner’s Law as applying only to voiceless fricatives. In general, sound changes will appear to have operated on the outputs of a previous sound change if one assumes in the first place that the previous sound change comes first: the key to justifying the relative chronology properly is to think about what alternative formulations of each sound change are required in order to make the alternative chronology (such alternative formulations can almost always be formulated), and establish the high relative unnaturalness of the sound changes thus formulated compared to the sound changes as formulable under the relative chronology which one wishes to justify.
If the PIE voiceless stops at some point became aspirated (which seems very likely, given that fricativization of voiceless stops normally follows aspiration, and given that stops immediately after obstruents, in precisely the same environment that voiceless stops are unaspirated in modern Germanic languages, are not fricativized), then Verner’s Law, formulated as voicing of obstruents in the usual environments, followed by Grimm’s Law formulated in the usual manner, accounts perfectly well for the data. A Wikipedia editor objects, or at least raises the objection, that a formulation of the sound change so that it affects the voiceless fricatives, specifically, rather than the voiceless obstruents as a whole, would be preferable—but why? What matters is the naturalness of the sound change—how likely it is to happen in a language similar to the one under consideration—not the sizes of the categories in phonetic space that it refers to. Some categories are natural, some are unnatural, and this is not well correlated with size. Both fricatives and obstruents are, as far as I am aware, about equally natural categories.
I do have one misgiving with the Verner’s Law-first scenario, which is that I’m not aware of any attested sound changes involving intervocalic voicing of aspirated stops. Perhaps voiceless aspirated stops voice less easily than voiceless unaspirated stops. But Verner’s Law is not just intervocalic voicing, of course: it also interacts with the accent (precisely, it voices obstruents only after unaccented syllables). If one thinks of it as a matter of the association of voice with low tone, rather than of lenition, then voicing of aspirated stops might be a more believable possibility.
My point here is not so much about the specific examples; I am not aiming to actually convince people to abandon the specific relative chronologies questioned here (there are likely to be points I haven’t thought of). My point is to raise these questions in order to show at what level the justification of the relative chronology needs to be done. I expect that it is deeper than many people would think. It is also somewhat unsettling that it relies so much on theoretical assumptions about what kinds of sound changes are natural, which are often not well-established.
Are there any relative chronologies which are very secure? Well, there is another famous Indo-European sound law associated with a specific relative chronology which I think is secure. This is the “law of the palatals” in Sanskrit. In Sanskrit, PIE *e, *a and *o merge as a; but PIE *k/*g/*gʰ and *kʷ/*gʷ/*gʷʰ are reflected as c/j/h before PIE *e (and *i), and k/g/gh before PIE *a and *o (and *u). The only credible explanation for this, as far as I can see, is that an earlier sound change palatalizes the dorsal stops before *e and *i, and then a later sound change merges *e with *a and *o. If *e had already merged with *a and *o by the time the palatalization occurred, then the palatalization would have to occur before *a, and it would have to be sporadic: and sporadic changes are rare, but not impossible (this is the Neogrammarian hypothesis, in its watered-down form). But what really clinches it is this: that sporadic change would have to apply to dorsal stops before a set of instances of *a which just happened to be exactly the same as the set of instances of *a which reflect PIE *e, rather than *a or *o. This is astronomically unlikely, and one doesn’t need any theoretical assumptions to see this.
Now the question I really want to answer here is: what exactly are the relevant differences in this relative chronology that distinguish it from the three more questionable ones I examined above, and allow us to infer it with high confidence (based on the unlikelihood of a sporadic change happening to appear conditioned by an eliminated contrast)? It’s not clear to me what they are. Something to do with how the vowel merger counterbleeds the palatalization? (I hope this is the correct relation. The concepts of (counter)bleeding and (counter)feeding are very confusing for me.) But I don’t think this is referring to the relevant things. Whether two phonological rules / sound changes (counter)bleed or (counter)feed each other is a function of the natures of the phonological rules / sound changes; but when we’re trying to establish relative chronologies we don’t know what the natures of the phonological rules / sound changes are! That has to wait until we’ve established the relative chronologies. I think that’s why I keep failing to compute whether there is also a counterbleeding in the other relative chronologies I talked about above: the question is non-well-formed. (In case you can’t tell, I’m starting to mostly think aloud in this paragraph.) What we do actually know are the correspondences between the mother language and the daughter language, so an answer to the question should state it in terms of those correspondences. Anyway, I think it is best to leave it here, for my readers to read and perhaps comment with their ideas, providing I’ve managed to communicate the question properly; I might make another post on this theme sometime if I manage to work out (or read) an answer that satisfies me.
Oh, but one last thing: is establishing the security of relative chronologies that important? I think it is quite important. For a start, relative chronological assumptions bear directly on assumptions about the natures of particular sound changes, and that means they affect our judgements of which types of sound changes are likely and which are not, which are of fundamental importance in historical phonology and perhaps of considerable importance in non-historical phonology as well (under e.g. the Evolutionary Phonology framework of Blevins 2004). But perhaps even more importantly, they are important in establishing genetic linguistic relationships. Ringe & Eska (2014) emphasize in their chapter on subgrouping how much less likely it is for languages to share the same sequence of changes than the same unordered set of changes, and so how the establishment of secure relative chronologies is our saving grace when it comes to establishing subgroups in cases of quick diversification (where there might be only a few innovations common to a given subgroup). This seems reasonable, but if the relative chronologies are insecure and questionable, we have a problem (and the sequence of changes they cite as establishing the validity of the Germanic subgroup certainly contains some questionable relative chronologies—for example they have all three parts of Grimm’s Law in succession before Verner’s Law, but as explained above, Verner’s Law could have come before Grimm’s; the third part of Grimm’s Law may also have not happened separately from the first).
 This quality difference exists in present-day English for sure—modulo secondary quality shifts which have affected these vowels in some accents—and it can be extrapolated back into seventeenth-century English with reasonable certainty using the written records. If we are ignoring the evidence of the written records, we can postulate that the quality differentiation between long /uː/ and short /ʊ/ was even more recent than the /uː/ > /ʊ/ shift (which would now be better described as an /uː/ > /u/ shift). But the point is that such quality adjustment can happen, as explained in the rest of the paragraph.
 There is a lot of literature on Grassman’s Law, a lot of it dealing with relative chronological issues and, in particular, the question of whether Grassman’s Law can be considered a phonological rule that was already present in PIE. I have no idea why one would want to—there are certainly PIE forms inherited in Germanic that appear to have been unaffected by Grassman’s Law, as in PIE *bʰeydʰ- > English bide; but I’ve hardly read any of this literature. My contention here is only that the generally-accepted relative chronology of Grassman’s Law and the devoicing of the PIE voiced aspirated stops can be contested.
 One should bear in mind some subtleties though—for example, *e and *a might have gotten very, very phonetically similar, so that they were almost merged, before the palatalization occured. If one wants to rule out that scenario, one has to appeal again to the naturalness of the hypothesized sound changes. But as long as we are talking about the full merger of *e and *a we can confidently say that it occurred after palatalization.)
 Actually, in practice we don’t know these with certainty either, and the correspondences we postulate to some extent are influenced by our postulations about the natures of sound changes that have occurred and their relative chronologies… but I’ve been assuming they can be established more or less independently throughout these posts, and that seems a reasonable assumption most of the time.
 I realize I’ve been talking about phonological changes throughout this post, but obviously there are other kinds of linguistic changes, and relative chronologies of those changes can be established too. How far the discussion in this post applies outside of the phonological domain I will leave for you to think about.
Blevins, J. 2004. Evolutionary phonology: The emergence of sound patterns. Cambridge University Press.
Minkova, D. 2013. A historical phonology of English. Edinburgh University Press.
Ringe, D. 2006. A linguistic history of English: from Proto-Indo-European to Proto-Germanic. Oxford University Press.
Ringe, D. & Eska, J. F. 2013. Historical linguistics: toward a twenty-first century reintegration. Cambridge University Press.