Inconstant consonants
This is one of the most intriguing tales of sound change I know: the tale of W.
Latin had a letter, V. It stood for the 'oo' sound we write as U. At the beginning of a word, it stood for the /w/ sound. So Latin in vino veritas was pronounced "in weeno weritas." Caesar's dispatch to the Senate, veni, vidi, vici, was pronounced "wehnee, weedee, weekee."
At the start of the early middle ages, long past the glory days of the western Empire, Latin speakers from all over began closing their lips from the pursed shape required for /w/ to two lips vibrating very close together: /v/. Eventually, people began pronouncing V with the bottom teeth against the upper lip -- as a labiodental fricative, rather than a bilabial one. And that's how we pronounce V today.
It was not inevitable that the switch to labiodental pronunciation would happen. And in some languages, it didn't. In French and Spanish, however, the bilabial fricative began to close to a semi-stop. Instead of making a longer vibrating sound, it now vibrated only for a moment and became hard to tell from the stop /b/. This is why we are often told as we learn these languages to pronounce Versailles as "bear-sigh" and vaca as "bah-kah." Indeed, the Gringos working with Mexicans took the Spanish word for cowboy, vaquero and turned it into their own "buckaroo."
Now, all the Germanic languages -- many of them still mutually intelligible in the early middle ages, used the sound /w/ prominently, and they had a rune to write the sound, called wynn: Ƿ. When the Anglo-Saxons began to acquire widespread literacy in the Latin alphabet, V was now pronounced /v/, so that was no longer available to them. So they just added the runic character wynn to the Latin alphabet, and were happy.
The Norman dialect of Old French had a lot of /w/ sounds, too, but they had a different spelling solution. They used the digraph GU. So "Guillaume" was pronounced, approximately, "William." "Guido" (later, Guy) they pronounced "Weedo." As Norman French pronunciation became replaced by Parisian French, GU became pronounced as /g/. Guillaume was now pronounced "Gee-yome" and Guido/Guy as "Gee-do/Gee" (hard /g/ in each case).
In place of the GU digraph, Norman scribes along with other continental scribes began experimenting with U, UU, and finally W to write the /w/ sound. And so it was as the other west Germanic tribes began to receive Christianity -- and with it, literacy -- Old High German and Old Low Franconian (the ancestor of Dutch) used W for /w/.
But then the same thing happened to Germanic speakers on the continent that had happened earlier to Latin speakers. /w/ began to turn into /v/. The spelling had already become fixed in languages like Old High German, but now instead of "water" they began to say "vasser." This is why W is pronounced /v/ in German and Dutch. However, Christianity and literacy came later to the speakers of Old Norse, and their switch from the sound /w/ to /v/ was already complete by the time their language was written down, which is why the Scandinavian languages use V to sound /v/ where German uses W.
English, though a Germanic language, didn't change with the other Germanic languages adjacent to it. In fact, in some of its pronunciation, it went the other way, turning /v/ into /w/! Take, for example, the two OE words hafoc (a bird of prey) and hlaford (a master who shares out bread to the workers). Old English F was pronounced in a voiced fashion -- as /v/ -- in medial positions, so these words would have been pronounced "ha-vokk" and "hla-vord." But over time, the medial /v/ sound began to widen from a bilabial fricative to a purse-lipped approximant: /w/, where wynn had not been before. So the bird of prey became pronounced "hah-wokk," spelled variously "hauk" or (our modern spelling) "hawk." As the H in halford ceased to be pronounced, The master dealing bread was pronounced "lah-word," and eventually, "lord." His wife, the hlæfdige, "hlav-dee-ya," became a "lady."
This is the way sound change works. Most people are unaware of it, but closely adjacent sounds begin to turn into each other as people speak rapidly and habitually. Spelling can follow sound, or it can fossilize old spellings which you have to remember to pronounce differently, and not according to the current rules.
Latin had a letter, V. It stood for the 'oo' sound we write as U. At the beginning of a word, it stood for the /w/ sound. So Latin in vino veritas was pronounced "in weeno weritas." Caesar's dispatch to the Senate, veni, vidi, vici, was pronounced "wehnee, weedee, weekee."
At the start of the early middle ages, long past the glory days of the western Empire, Latin speakers from all over began closing their lips from the pursed shape required for /w/ to two lips vibrating very close together: /v/. Eventually, people began pronouncing V with the bottom teeth against the upper lip -- as a labiodental fricative, rather than a bilabial one. And that's how we pronounce V today.
It was not inevitable that the switch to labiodental pronunciation would happen. And in some languages, it didn't. In French and Spanish, however, the bilabial fricative began to close to a semi-stop. Instead of making a longer vibrating sound, it now vibrated only for a moment and became hard to tell from the stop /b/. This is why we are often told as we learn these languages to pronounce Versailles as "bear-sigh" and vaca as "bah-kah." Indeed, the Gringos working with Mexicans took the Spanish word for cowboy, vaquero and turned it into their own "buckaroo."
Now, all the Germanic languages -- many of them still mutually intelligible in the early middle ages, used the sound /w/ prominently, and they had a rune to write the sound, called wynn: Ƿ. When the Anglo-Saxons began to acquire widespread literacy in the Latin alphabet, V was now pronounced /v/, so that was no longer available to them. So they just added the runic character wynn to the Latin alphabet, and were happy.
The Norman dialect of Old French had a lot of /w/ sounds, too, but they had a different spelling solution. They used the digraph GU. So "Guillaume" was pronounced, approximately, "William." "Guido" (later, Guy) they pronounced "Weedo." As Norman French pronunciation became replaced by Parisian French, GU became pronounced as /g/. Guillaume was now pronounced "Gee-yome" and Guido/Guy as "Gee-do/Gee" (hard /g/ in each case).
In place of the GU digraph, Norman scribes along with other continental scribes began experimenting with U, UU, and finally W to write the /w/ sound. And so it was as the other west Germanic tribes began to receive Christianity -- and with it, literacy -- Old High German and Old Low Franconian (the ancestor of Dutch) used W for /w/.
But then the same thing happened to Germanic speakers on the continent that had happened earlier to Latin speakers. /w/ began to turn into /v/. The spelling had already become fixed in languages like Old High German, but now instead of "water" they began to say "vasser." This is why W is pronounced /v/ in German and Dutch. However, Christianity and literacy came later to the speakers of Old Norse, and their switch from the sound /w/ to /v/ was already complete by the time their language was written down, which is why the Scandinavian languages use V to sound /v/ where German uses W.
English, though a Germanic language, didn't change with the other Germanic languages adjacent to it. In fact, in some of its pronunciation, it went the other way, turning /v/ into /w/! Take, for example, the two OE words hafoc (a bird of prey) and hlaford (a master who shares out bread to the workers). Old English F was pronounced in a voiced fashion -- as /v/ -- in medial positions, so these words would have been pronounced "ha-vokk" and "hla-vord." But over time, the medial /v/ sound began to widen from a bilabial fricative to a purse-lipped approximant: /w/, where wynn had not been before. So the bird of prey became pronounced "hah-wokk," spelled variously "hauk" or (our modern spelling) "hawk." As the H in halford ceased to be pronounced, The master dealing bread was pronounced "lah-word," and eventually, "lord." His wife, the hlæfdige, "hlav-dee-ya," became a "lady."
This is the way sound change works. Most people are unaware of it, but closely adjacent sounds begin to turn into each other as people speak rapidly and habitually. Spelling can follow sound, or it can fossilize old spellings which you have to remember to pronounce differently, and not according to the current rules.