As you can imagine, there is no rigid rule here, each relative or friend finding one nickname or another as they feel it to sound right, which is hard to sum up as strict rules.

So, looking at existing usages, one can note :

• sometimes one use just one of the two parts as shortcut (so, yes, Jean for Jean-Jacques is rare but happens), often the first one of the two. This will only be used when noone has this shortcut as his real name (i.e. if there is already someone named Jean, this won’t be Jean-Jacques‘s nickname…).

• sometimes, the last part of the composite name is shortened, leading to Jean-Phi for Jean-Philippe for example.

• there’s also the possibility of using initials, quite used for some names, like :

  • JC (pronounced as capital letters, jicé) >>> Jean-Christian / Jean-Claude / Jean-Charles

  • JP (jipé) >>> Jean-Pierre / Jean-Patrick / Jean-Philippe

  • JB (jibé) >>> Jean-Bernard / Jean-Benoit / Jean-Brice / Jean-Baptiste

  • JF (ji-ef) >>> Jean-François / Jean-Frédéric

  • JD (jidé) >>> Jean-Damien

  • etc.

• and probably some other custom transformations of course, I’ll add some if any other pop up in mind later

Prénoms composés are a single first name, not a first and middle name as foreigners often believe.

I find it very annoying when called “Jean” while my first name is “Jean-Louis”.

French people do not use nicknames that much, and at least much less than Americans and in any case, it is almost never the first name of a compound one, especially when it is “Jean” which used to be very common case.

It is just like you don’t refer to “San Francisco” as “San” or “New-York” as “New”.

It might be the last one though. I know people named “Pierre-Cyril” and “Marie-Ghislaine” who used to be called “Cyril” and “Ghislaine”.

In any case, don’t try to guess a nickname from someone’s first name, there is no generic rule. Never use a nickname unless you know that other people already do it.

You got good answers from everyone, but keep in mind that it’s less common than it was.

I was born in 1995 and I have never got any classmate with a composed name. But my father has one. If you meet someone named Jean-Pierre, just call him Jean-Pierre unless you’re close.

In France, names often depends on the age of the person, it will be common to find someone named Thierry or Jean-Philippe (30 years old +) but it will be pretty rare to find one who is less than 20.

In these 10 last years, many people named their children with a ‘short name’ as Léo, Lou etc. But composed names tend to disappear, at least for this generation.