Questions: Qui, qui est-ce qui = Who ?

Only words ending in "e" contract. 

Mainly it's words ending in "e", but also "la" and "si".

The rules are explained here:

https://kwiziq.learnfrenchwithalexa.com/revision/glossary/contraction/l-elision-elision

But si does not contract if the following vowel is an "e". It only contracts in cases such as si+il = s'il or s+ils = s'ils.

If you want a more complete summary on elisions, you can check, e.g.,
here:https://www.lawlessfrench.com/pronunciation/elision/

Hi Lewis,

 In answer to your question logically, it is probably to differentiate the meanings which would be lost if you could elide both vowels - 

Qu'est-ce -que ...? = What is ...?

Qui est-ce que ...? = Who is ...?

Hope this helps.

I would translate "Qui est-ce qui m'a traité" as "Who treated me." I am not familiar with traiter in the meaning of to insult. -- Chris (not a native speaker).

Bonjour Donald and Chris !

Indeed, the verb "traiter" can be used to mean "to insult".

Here's where that comes from: the original sentence is: 
traiter [quelqu'un] de [quelque chose] = to call [someone] [something] 

but in speech, it's become common to imply the insult itself and to use "traiter" on its own, i.e. to call [someone], meaning to insult [someone].

I hope that's helpful!
Bonne journée !

Bonjour Stuart, From the lesson: "Qui means who but there is a longer form in which 'qui' appears twice: qui est-ce qui" In other words, they mean exactly the same thing, and qui is more common. I would guess that the question you're referring was multiple answer, meaning that you have to choose all of the possible right answers in order for that question to be marked as correct.

Bonjour Susan, "Qui est-ce ?" is a closed question in itself, meaning "Who is it?", you cannot use it to ask "who is it [that....]". It's in that case that you'll use either "Qui [...]?" or "Qui est-ce qui [...]?". I hope that's helpful. À bientôt !

Merci!! Je comprends maintenant!