The concept of "mother" in linguistics

I began drafting this post around Mother's Day, which we recently observed, but got distracted by other things.  This is an old topic that I've been thinking about for years.  Namely, I've long been intrigued by the use of mǔ 母 ("mother") in linguistic terms, such as zìmǔ 字母 ("letter", lit., "character mother") (e.g., sānshíliù […]