Why do classic names have diminutives / nicknames that change the initial?

There is - or was - a tendency in English to take very common names and for a nickname to develop that changed out the initial. To wit:

Richard —> Rick—>Dick. In the past (until it started to be common as slang for a certain unnamed part of the male anatomy???), it was common to go by “Dick”. Where did the “D” come from?

Robert—> Rob—>Bob. I could see how the terminal “B” could cause an alliteration (similar to Ann(e) becoming “Nan”). But in the Middle Ages, there was also the nickname “Hob”. Why ever would one replace the “R” with an “H”.

The name “Mary” has gone through a sequence of modulations. In the past, a common nickname for Mary was “Moll”, which was further changed into “Molly”. But from that we got “Poll” and “Polly”. Why was the “P” necessary?

I think it’s just that some names were so common that you needed a lot of different nicknames for them, so everyone with that name in a community could be called something different. There are only so many nicknames you can make while keeping the same letter.

Wikipedia suggests it’s an example of rhyming slang.

ISTM a lot of groups developed rules about naming, such that the first boy was named for paternal grandfather, second named for [some other carefully specified relative], etc. Same concept for baby girls.

The net result of which is a very large number of people deliberately reusing a very short list of names.

AIUI these practices have just about died out in the USA. There are a few holdout ethnicities or religious sects that (generally) follow their traditional patterns. But not many different groups and they represent a small fraction of the current population.

So did Margaret turn to Meg and then Peg?

Elizabeth, Elspeth, Betsy, and Bess,
They all went together to seek a bird’s nest;
They found a bird’s nest with five eggs in,
They all took one, and left four in.

This covers some of the most common English ones, you can see a few patterns.

It’s been noted it’s extremely rare to hear of a person under (60? 70?) Named ‘Richard’ to have a nickname of ‘Dick.’

In the 1960s, there was a football coach ‘Dick Wood.’ I doubt at the time anyone gave it a second thought, but nowadays it would probably garner more than a few snickers.

“M” and “P” are both bilabial consonants, so that switch doesn’t seem completely arbitrary to me.

I just let out quite a loud snicker.

I know one - but he’s 80 (amusingly, when he was 10 or so, he met Dick Nixon (I’ve seen the photo))

Agree. If Mary is called and 8 people appear it’s alittle confusing.

So nicks were used.

I am the same. I use my middle name as my defacto name.
It’s Rebecca. There’s 20 more in my extended family. A GG name.
Of course it was babified to Becky. I was that until I forced folks to call me Beck in my later teens.

Works sometimes.

Nicknames piss me off. Well, my own does.

I knew a guy named Dick Head. You have to wonder what his parents were thinking.

Dick Trickle. Dick. Trickle.

As a college kid I worked for a guy named Ronald McDonald. He was a kid when the hamburger clown was invented. I don’t think that improved his passage through school.

Perhaps Dick Head’s parents had the same outcome for the same reason. Evolving slang moved the name from “short and punchy” to “raunchy punchline” underneath the kid.

This happened to my brother. He has the same name as a very famous person. But my parents don’t have the blame for this; the person didn’t become famous until several years after my brother was born and named.

Now that you mention it, I have met a Richard Head. We were not that familiar, though, so it never came up.

Okay, I’ll admit that Dick Head generally used the name Rich. But he was aware of the joke and would call himself Dick Head as a joke.

I once met an English guy who presented himself as Dick Little. This was in the 90’s. I got to know him slightly. He was running A (dreadful English ex-pat) bar in the south of Spain. Turns out he really was named Richard Little. Showed me his passport.

I sincerely doubt his tale of having to mates in the military going by Dick Tiny and Dick Small.

Also, there’s this:

Yeah. I used to work with a guy a couple years older than I. One William F. Gates, who went by Bill. He wasn’t the software magnate, but he was about the same age. He too was named long before “Bill Gates” became a household name.

What was funny is that when we both were younger, the famous Bill Gates and I looked very much alike. Cousins for sure, maybe brothers or even fraternal twins. We’ve since aged differently. My co-worker Bill Gates looked nothing like famous Bill Gates.

Anyhow, back in the day Bill would introduce himself over the PA as the pilot and then people would see me standing there welcoming or goodbyeing the crowd. Got more than a few double-takes with that one.