> * the pronunciation of descendent languages, especially when there are lots of them, or lots of different dialects
It's worth calling this out as being one of the key elements of historical linguistics. Establishing genetic relations between languages requires proposing systematic sound shift laws that can explain why cognates sound the way they do, and this means that cognates in modern languages may not bear much resemblance to each other (English five and Sanscrit pankan are cognate yet share 0 sounds!). For example, there's a rule in the Germanic languages that shifts /k/ to /h/, so words like Latin "centum" instead become English "hundred" or "canis" to "hound" [1].
Now these pronunciation shifts often have caveats in them, such as shifting only before certain kinds of vowels or consonants. These restrictions can give you some clues as to why certain words seem to undergo a change while other words with seemingly similar pronunciations didn't. In Proto-Indo-European, this leads to the notion that there are several consonants (specifically, laryngeals) which are no longer present in any modern Indo-European language but whose existence in the original is responsible for sometimes shifting vowels that otherwise appear somewhat anomalous in descendant languages.
[1] To be clear, Latin is not the initial word and English is not the final word. I'm just using Latin to illustrate a word closer to the original Proto-Indo-European pronunciation and English to illustrate what the Proto-Germanic pronunciation shifts towards.
It's worth calling this out as being one of the key elements of historical linguistics. Establishing genetic relations between languages requires proposing systematic sound shift laws that can explain why cognates sound the way they do, and this means that cognates in modern languages may not bear much resemblance to each other (English five and Sanscrit pankan are cognate yet share 0 sounds!). For example, there's a rule in the Germanic languages that shifts /k/ to /h/, so words like Latin "centum" instead become English "hundred" or "canis" to "hound" [1].
Now these pronunciation shifts often have caveats in them, such as shifting only before certain kinds of vowels or consonants. These restrictions can give you some clues as to why certain words seem to undergo a change while other words with seemingly similar pronunciations didn't. In Proto-Indo-European, this leads to the notion that there are several consonants (specifically, laryngeals) which are no longer present in any modern Indo-European language but whose existence in the original is responsible for sometimes shifting vowels that otherwise appear somewhat anomalous in descendant languages.
[1] To be clear, Latin is not the initial word and English is not the final word. I'm just using Latin to illustrate a word closer to the original Proto-Indo-European pronunciation and English to illustrate what the Proto-Germanic pronunciation shifts towards.