Q:

What is ”A regular sound change” in old English?

I found this term in Herisson’s answer to the question: How did “Pappa” become “Pope”?
I am wondering what a “regular sound change” is and why would it mean people stopped saying Pappa and began saying Pope?

A:

A 'regular sound change' is a term for a concept from linguistics. It means that a particular sound consistently (or regularly) changes to another sound under the same circumstances. That is, there is a rule (a regular change) that specifies the change. For example, as herisson said:

A regular sound change turned Old English ā [ɑː] into Middle English [ɔː], a long back mid-open rounded vowel that was generally written with the letter o. And Middle English [ɔː] regularly develops to the modern English "long o" sound, which is spelled the same way.

In other words, in Old English in whatever circumstances there appeared an 'ā', then for the same circumstances, the same sequence in Middle English would replace the 'ā' with an 'ɔː'. For example,  /pā pa/ -> /pɔː pa/.
(Note: I am unsure of whether Middle English had the 'silent e' change yet)

