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As we know, the English language doesn't have the /x/ phoneme anymore (at least in an everyday kind of context*) and the sound seems to have been dropped in many words, such as in light or eight.

However, consider the following words: laugh, tough, rough and cough. It strikes me as very odd that what was once a /x/ would turn into /f/; it seems much more likely to me that it would have shifted to /h/, /ʃ/ or even /g/ — but definitely not to a fricative at the front.

My question is: Has it been attested that /x/ shifted to /f/ in certain situations ? Is there an explanation for this phenomenon?

laugh is lachen in both German and Dutch with a /x/-like sound; rough is ruig in Dutch and ruuch in Swiss German (both with /x/). So there is some relation between /x/ and /f/, but it perplexes me.

(On an irrelevant note, I also notice that shaft is Schacht in German, but the whole ft vs. cht at the end of words seems to be more complicated and possibly independent from the /f/ in the words I mentioned in the second paragraph. There are some Dutch-German pairs where they systematically differ: {kracht, Kraft}, {lucht, Luft}, {stichting, Stiftung}. This is odd, too, but not in the scope of this question — or of this site for that matter.)

* disregarding all annoyed teenagers who use "ugh" excessively ;)

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Are you certain that those words used to use /x/ ? That isn't at all clear to me. – Rory Alsop Jul 18 '14 at 14:09
@RoryAlsop rough comes from Old English ruh which was pronounced /ruːx/. Can't find definitive evidence for laugh or cough. The word had hh in Old English and gh in Middle English from what I looked up. I don't know how they're pronounced, though. I just know the words I mentioned have cognates with /x/ and there seems to be some pattern. – ljacqu Jul 18 '14 at 14:14
/x/ to /f/ is a shift from velar to labial which requires very little practical change (start with a continuous /x/ sound and bring the lower lip closer to your upper teeth). this has some more info (note that in some languages/cases the shift goes the other way round) – msam Jul 18 '14 at 14:51
Native speakers of different languages hear different phonemes as close to each other. For example, a German speaker once asked me why English-speaking people replace /x/ with /k/ (consider loch and Bach), since they sound nothing like each other. The best answer I could give him was that they sounded similar to English speakers, which didn't satisfy him at all. Presumably, /f/ was the closest-sounding phoneme to /x/ for English speakers when this change happened in Middle English. – Peter Shor Jul 18 '14 at 15:05
Och, I think it's just sassenachs who've lost their /x/ – High Performance Mark Jul 18 '14 at 16:12
up vote 11 down vote accepted

Not really answering "why" but here is "how":

This is called labial-velar shift and occurs in many other instances. Both /x/ and /f/ are fricatives and the change required to go from the velar /x/ to the labial /f/ or the other way round is pretty small (to make an /f/ start with an /x/ and touch lower lip to the upper teeth for, alternatively make an /f/ and open your mouth, you should end up with an /x/ ).

old english/old high german ruh -> english rough / german rau(this changed in a different manner in the case of german)

old english/old high german hlahhan -> english laugh / german lachen

it can also occur the other way round:

old high german luft -> dutch lucht

Much more detailed information can be found in this paper (as well as, I'm sure, other sources)

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Interestingly, in Swiss German we still say ruuch for High German rau (English rough). – ljacqu Jul 19 '14 at 5:23

Because of labialization: the influence of another sound pronounced with the lips caused the fricative /x/ to change into a fricative pronounced with the lips.

In all of the cases where /x/ turned to /f/ in English, you'll notice the word is spelled with a "u" before the "gh" digraph. This is not a coincidence. During the Middle English period, the /x/ sound developed a "high vowel" sound before it, either [i̯] or [u̯], and this is reflected in the spelling – you'll almost always see the letter "i" or "u" before a "gh" that represents historical /x/. (These were not full syllables, but glides.) Which glide was inserted depended on the preceding vowel; after back vowels like /a/, /o/ and /u/, a [u̯] was inserted, and after front vowels like /e/ an [i̯] was inserted. (For /i/, as far as I can tell, the vowel was simply lengthened.)

After this process of glide insertion, the glides were later eliminated in various ways. In most cases, they merged with the preceding vowel, and the /x/ was later lost. This is what happened in words like "caught" or "ought." But in other cases, the glide merged with the following /x/ instead. The [u̯] glide can be classified as a labial sound, made with the lips, and it imparted this lability to the fricative /x/, resulting in a labiodental fricative /f/.

It's believed that an intermediate step in this process was the shifting of the labial glide from before the fricative /x/ to simultaneously with it, or even after it.

In the development of English [f] from earlier round vowels followed by [x], a crucial step was evidently a shift such as [ʷx] > [xʷ], in which labialization is realigned with the end of the fricative.

Origins of Sound Change: Approaches to Phonologization, by Alan C. L. Yu, 2013

A piece of evidence that the glide disappeared in the cases where /x/ became /f/ is that words like "laugh" have the "short a" sound rather than the usual sound associated with the digraph "au."

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+1 Informative and well written. Thanks for the link ... – Araucaria Jul 10 '15 at 10:04

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