Is Jack a nn for Jonathan...

[CheerfulYank]

or just John?

Just John, I think.

Traditionally it's a nn for John.

I don't see why it couldn't be used for Jonathan though (especially as Jackathan might be a step too far....)

Jack derived solely as a nn for John.

In medieval times, the name John was altered in some of the Germanic languages, to be Jankin (as this sounded better in Germanic tongues).

This in turn inspired the name Jackin in England. Jackin started to be used as a nn for John in England and that got streamlined to Jack.

Therefore, strictly speaking, Jack has no connection to Jonathan at all.

I'm sure Jacks exist whose 'proper' name is Jonathan. That's the good thing about nicknames, there aren't really any rules. I mean, who's going to argue with you? It's not like you're saying 'meet Jack, short for Timothy', the link is pretty clear.

Just John AFAIK.

Most Jonathans I know spell their nn Jon. Jonny is also short for Jonathan.
You can choose whatever nickname you like though - if you want to use Jack then go for it!

I have a Jonathan (only Jonathan - he doesn't like it shortened!) Jonny, Jonty and Jon are the comman ones - but I like Jack and think it'd fit fine

joth or joss are nn for jonathan too

Why would you even want to shorten a lovely name (Jonathan, John/Jonny) to THE most popular boys name... I'd stick with Jonny for when he's young - so cute and much nicer than Jack imo.

Rachel Jack is not the most popular boy's name in America. We're overrun with Jakes, however. :)

I like Jonty short for Jonathan. Know a few of those.

Nicknames are different from diminutives. I had an Uncle, Albert Thomas, who was always known as Bill because of his resemblance to a cartoon character called Bill. Using that line of reasoning you can call a Jonathan Jack whether anyone else does or not. Our Jonathan was known as Billy for quite a while for similar reasons.